Add a radio box to the Sitewide Sale Landing Page that dynamically pulls in that sale's discount code and applicable levels.

pmpro_sitewide_sales_radio_select.php

<?php
/*
 * Add a radio box to the Sitewide Sale Landing Page that dynamically pulls in that sale's discount code and 
 * draws a radio selection for each level the code applies to.
 * Shows the modify level cost text based on the applied discount code for each level.
 */

/**
 * We need to set $_REQUEST['level'] early on our
 * sitewide sales pages to support other customizations.
 */
function set_request_level_on_sitewide_sales_pages() {
    // Get the current page object.
    $post_id = url_to_postid( $_SERVER['REQUEST_URI'] );
    
    // Does the current page have an associated Sitewide Sale?
    $sitewide_sale_id = get_post_meta( $post_id, 'pmpro_sws_sitewide_sale_id', true );

    // The current page is a Sitewide Sale Landing Page.
    if ( ! empty( $sitewide_sale_id ) ) {
        $default_level_id = PMPro_Sitewide_Sales\includes\classes\PMPro_SWS_Landing_Pages::get_default_level( $post_id );
        
        // If the level was passed in the URL, return that level.
        if( ! empty( $_REQUEST['level'] ) ) {
            return;
        }
        
        // If no level was passed in the URL, specify the default level from the Sitewide Sale.
        if ( ! empty( $default_level_id ) ) {
            $_REQUEST['level'] = $default_level_id;
            define( 'IS_SITEWIDE_SALE', true);
        }        
    }
}
add_action( 'init', 'set_request_level_on_sitewide_sales_pages', 1 );

/**
 * Hide the PMPro pricing box at checkout and replace with our own.
 *
 * Requires Paid Memberships Pro v1.9.5.6+ and the Sitewide Sales Add On v1.0+
 *
 * Add this code below to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function maybe_add_choose_level_section( $include_pricing_fields ) {
    global $wpdb, $pmpro_level;

    // Make sure Sitewide Sales Add On is active.
    if ( ! class_exists( 'PMPro_Sitewide_Sales\includes\classes\PMPro_SWS_Settings' ) ) {
        return $include_pricing_fields;
    }

    // Hide the default pricing box.
    if ( PMPro_Sitewide_Sales\includes\classes\PMPro_SWS_Settings::is_active_sitewide_sale_landing_page() ) {
        $include_pricing_fields = false;
        
        // Get the current page's associated Sitewide Sale and discount code.
        $sitewide_sale_id = get_post_meta( get_queried_object_id(), 'pmpro_sws_sitewide_sale_id', true );
        $discount_code_id = get_post_meta( $sitewide_sale_id, 'pmpro_sws_discount_code_id', true );

		$sqlQuery = $wpdb->prepare("
			SELECT l.id
			FROM $wpdb->pmpro_membership_levels l
			LEFT JOIN $wpdb->pmpro_discount_codes_levels cl
			ON l.id = cl.level_id
			WHERE cl.code_id = %d",
			$discount_code_id
		);
        
		$discounted_levels = $wpdb->get_results($sqlQuery);
        
		$discounted_level_ids = array();
		foreach ( $discounted_levels as $level ) {
            $discounted_level_ids[] = $level->id;
        }        
        
        // Show our own radio select and info.
        ?>
        <div id="pmpro_levels_select" class="pmpro_checkout">
            <h3><span class="pmpro_checkout-h3-name">Select a Level</span></h3>
            <div class="pmpro_checkout-fields">
            <?php 
                foreach ( $discounted_level_ids as $discounted_level_id ) { 
                    
                    $level = pmpro_getLevel( $discounted_level_id );

			        $sqlQuery = $wpdb->prepare("
                        SELECT l.id, cl.*, l.name, l.description, l.allow_signups 
                        FROM $wpdb->pmpro_discount_codes_levels cl 
                        LEFT JOIN $wpdb->pmpro_membership_levels l 
                        ON cl.level_id = l.id 
                        LEFT JOIN $wpdb->pmpro_discount_codes dc 
                        ON dc.id = cl.code_id 
                        WHERE dc.id = %d 
                        AND cl.level_id = %d 
                        LIMIT 1",
                        $discount_code_id,
                        $level->id
                    );

			        $level = $wpdb->get_row($sqlQuery);
                                        
			        //filter adjustments to the level
			        $level = apply_filters("pmpro_discount_code_level", $level, $discount_code_id);

					//apply filters
					$level = apply_filters("pmpro_checkout_level", $level);
                    ?>
                    <div class="pmpro_checkout-field pmpro_checkout-field-radio pmpro_checkout-field-level-radio" style="cursor: pointer;">
                        <input id="levelradio-<?php echo $level->id; ?>" type="radio" name="level" value="<?php echo $level->id; ?>" <?php checked( $pmpro_level->id, $level->id ); ?> />
                        <strong class="pmpro_checkout-field-level-name"><?php echo $level->name; ?></strong>
                        <span class="pmpro_checkout-field-level-cost"><?php echo pmpro_getLevelCost( $level, false, true ); ?></span>
                    </div>
                <?php } ?>
                <script>
                    jQuery('.pmpro_checkout-field-level-radio').click(function() {
                        jQuery(this).addClass('selected').find("input[type='radio']").prop('checked', true);
                    });
                </script>
            </div>
        </div>
        <?php
    }
    
    return $include_pricing_fields;
}
add_filter( 'pmpro_include_pricing_fields', 'maybe_add_choose_level_section' );

This gist is not compatible with the Sitewide Sales plugin and will need to be updated to work with that plugin. You can use a modified version of this recipe instead: https://gist.github.com/kimcoleman/0146529d87c2950e43b23c9f4d2dfa71