write an issue as XML to a file... it's the same XML as shown by Export > XML

IssueAsXml.groovy

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.config.properties.APKeys
import com.atlassian.sal.api.net.Request
import com.atlassian.sal.api.net.TrustedRequest
import com.atlassian.sal.api.net.TrustedRequestFactory
import com.onresolve.scriptrunner.runner.customisers.PluginModule
import groovyx.net.http.URIBuilder

@PluginModule
TrustedRequestFactory trustedRequestFactory

final issueKey = 'SSPA-1'
def issue = ComponentAccessor.getIssueManager().getIssueByCurrentKey(issueKey)
assert issue

def loggedInUser = ComponentAccessor.jiraAuthenticationContext.loggedInUser
def baseUrl = ComponentAccessor.applicationProperties.getString(APKeys.JIRA_BASEURL)
def host = new URIBuilder(baseUrl).host
def xmlEndpointUrl = "/si/jira.issueviews:issue-xml/${issue.key}/${issue.key}.xml"

def request = trustedRequestFactory.createRequest(Request.MethodType.GET, "${baseUrl}$xmlEndpointUrl") as TrustedRequest
request.addTrustedTokenAuthentication(host, loggedInUser.username)
def result = request.execute()

new File("/tmp/${issue.key}.xml").withWriter { w ->
    w.write(result)
}