Some engines will tolerate a little more timing. home built water injection(water vapour) might help?
The biggest problem is breathing and a non cross flow head. The intake and exhaust are very restrictive.The cam timing is lazy.The carb is small.The air cleaner is restrictive. Fix any of these problems with what ever you can make or rob from other cars.Maybe a KnN air filter will give you 2 or 3 hp?