Migrate SAP CRM On-premise Customization to SAP S/4 Service Onpremise

Our existing SAP CRM solutions is customized heavily with several screens, Fields and assignments blocks within the area of Lead, Opportunity, Quote and Activity Management, Service ticket and Service contracts.

Does SAP provide any tool to migrate this custom code from CRM On-premise solution to S/4 HANA Service On-premise.

Does this need to be re- developed again in the SAP S/4 Service On-premise solution ? Re-developing this custom code is very time taking, just wanted to check if SAP as any best practice or easy way of doing this.

Thank you,

Goutham