You need to do something to the counterweight system. These have a habit of self-destructing at higher rpm taking the crankcase with them. There are different approaches. The simplest is a billet dogbone to keep the counterweight moving where it should. The next is to get rid of the counterweight altogether and add fixed counterweights directly to the crankshaft. Finally, you can use a billet crankshaft that has these weights forged into it. Apart from the billet crankshaft you can't buy the other parts new unfortunately, so you need to try and source them on sites like this one. Needless to the say, the billet crankshaft is a very expensive option. So that keeps the engine together but really doesn't add much to the performance other than allowing it to rev faster when you've improved some other areas.

For performance it's all about getting more air through the engine by letting it breath better. So the port and polishing you did helps in this regard as will a velocity stack on the carburettor intake and a freer flowing exhaust. Internally, you can shave the eyebrows between the valves and the cylinder, I suggest you find some videos on how to do this. Next there's higher lift and duration camshafts and stronger valve springs. I'm not aware if it is possible to fit larger intake valves - I don't think so. You'll need to fit stiffer, probably duplex valve springs so they don't float at high speed. Combined with the above you should consider adjusting the spark timing to advance it more so you can get more power at higher rpm.

The next thing you could consider is increasing the compression by shaving the head or using a 11hp head on a 12.5hp motor. On this point, especially if changing cams watch the clearances to the head.

I haven't mentioned anything about general rebuilding to ensure the clearances are optimised but that's a whole subject on it's own.

If you?re still looking to sell parts of this motor I would certainly be interested in the counterweights and perhaps the carb.