If the engine mounts in the standard configuration - on the side engine lugs, I’d change the engine for a better replacement than the Toki/Shuwa version. Some of these engines mounted from the bottom of the crankcase - of which I’ve only seen from these Toki/Shuwa Engines. If that’s how the engine is mounted, stick with that style unless you want to change the engine mount to fit a modern engine. Some pictures of the engine and mount would be helpful IMO.