You can't ad hock build something that works well.
Buy a Predator to build into an engine and keep the one you have now functional so you have something to ride.
Try a 4 deg key ( file your own if you have a vice ).
Get a timing light and find out exactly what your starting with and try and get to around 25 ( but no more that is the upper limit for something to still run well at low speed ).
Out of the box all these engines are lean, but your going to have to richen it up and within the limits of your carb and exhaust.
How much depends on a lot of things.
Find a 14 cc head and bolt that on.
Its a cheap easy find at most small engine shops and it will build power at all RPM ranges and still allow you to use a governor and stock internals.
You might luck out and find a GXV160 mower engine around ( commercial Honda mower ) this has a flywheel that is as strong as billet, lighter than stock and has about 5 deg of timing advance built in ( hard to screw up ).
Header, filter kit, timing, 14cc head, GXV flywheel all this is cheap stuff you can find locally or make yourself, you can even drill your own jets ( but I suggest you get some scrap carbs to practice on ).
The idea with these suggestions is they are relatively easy bolt on changes that can be undone easily too.