I think you might be better off just to do a little bit of porting and surfacing of the head.
If you don't run an expansion chamber you can get away with pretty high compression.
So here's some good specs for your stock exhaust and 15 mm carb.
So this is what you should do get a intake manifold that has a smooth bend in it.
Your piston Port timing should be about 135° to 138 degrees.
Shimmy your cylinder up 1 mm to raise the transfer ports.
But if you're patient want to open up the transfer ports you want to get those in about a 120 to a 122° range.
And your exhaust Port timing in the range of 160° to 164°.
That should get you close to about three and a half horsepower and you should be able to cruise close to 40 mph with your stock exhaust.
And set your timing at 18°, you might even be able to get away with 19° since you're not running a pipe just watch your temps (edited)