Find the leak on the Kawasaki FH and fix it. If it can’t be fixed and is a small leak, then keep oil topped off and run it. Lots of small engines leak oil. You don’t do an engine swap over a oil leak, unless it is gushing out.
Where is it leaking? Out the top oil seal? If it is replace the engine the top main bushing is worn out. You can replace the seal but it will blow it right back out.