Probably a good idea to retain the original. You can be sure that the motor will run well in that configuration.

Yep, it's bulky. Yep, it's quite heavy ... but, you need the nose-weight, don't you?

I had a 40-odd cc Kawasaki that was "intact". Bloody lovely engine. And, of course, the Zenoahs are all great with their OEM ignition.

Maybe you'll happen across a more elegant engine in the future, and you will be able to swap.