Simple Visualforce template for displaying converted lead records. Once a lead is converted the standard detail page is no longer accessible, must use reports or visualforce.

LeadConvertedViewController.java

/**
 * https://twitter.com/Joy_SH/status/738087166547398656
 * https://help.salesforce.com/apex/HTViewSolution?id=000175908&language=en_US
 */ 
public with sharing class LeadConvertedViewController {

    public Lead record { get; private set; }

    public LeadConvertedViewController( ApexPages.StandardController stdController ) {
        this.record = (Lead) queryRecord( stdController.getId() );
    }

    private SObject queryRecord( ID recordId ) {

        // https://douglascayers.wordpress.com/2015/10/08/salesforce-how-to-select-all-fields-with-soql-in-apex/

        DescribeSObjectResult describeResult = recordId.getSObjectType().getDescribe();

        List<String> fieldNames = new List<String>( describeResult.fields.getMap().keySet() );

        String query =
          ' SELECT ' +
              String.join( fieldNames, ',' ) +
          ' FROM ' +
              describeResult.getName() +
          ' WHERE ' +
              ' id = :recordId ' +
          ' LIMIT 1 '
        ;

        List<SObject> records = Database.query( query );

        return ( records.size() > 0 ) ? records[0] : null;
    }

}

LeadConvertedViewPage.html

<apex:page standardController="Lead" extensions="LeadConvertedViewController" readOnly="true">

    <apex:pageBlock>

        <apex:pageBlockSection columns="1">

            <!--
              Unfortunately, you have to build out the page yourself
              to show chatter feed, fields and any related lists.
              You cannot use the <apex:detail> tag that would naturally
              render the user's appropriate page layout, will get Insufficient Privileges.
            -->

            <apex:outputField value="{!record.firstName}"/>
            <apex:outputField value="{!record.lastName}"/>
            <apex:outputField value="{!record.company}"/>

        </apex:pageBlockSection>

    </apex:pageBlock>

</apex:page>

To make the converted lead data accessible by your users, you can create a lead with converted info report as described here: https://help.salesforce.com/apex/HTViewSolution?id=000175908&language=en_US

If you want to add some flavor and provide a visualforce page to display other data, related lists, etc. Then from the report have a link to the visualforce page (the link can be defined as a custom URL field on the lead and added to the report).