I've been running and modding these motors for about 10 years
now. I have 2 running and two on the shelf right now.
According to PSI Performance whom I bought race pipes from some
years ago, the stock 700 put out 120hp. With their pipes it
went up to 128hp. With their pipes and extensive cylinder porting they got 135hp. That seems to be about tops for this motor without using custom built cylinders. PSI did make some custom cylinders for this, and I think they got about 150+hp out of it, but thats just my foggy memory. That was also with their own custom built BigAir carbs at 500 bucks apiece.
Wiseco/US Chrome developed a 790cc bigbore kit for the motor.
To work properly the heads need to be modified and the exhaust port reshaped. I have these kits installed on 2 of my sleds.
One has extensively ported cylinders with extra exhaust boost
ports added, to make it like the Black Magic race jugs. I don't
think I am getting much more power than stock hp with all my
mods. I'd like to dyno it sometime. All the mods are very incremental. You need to tweak everything from the engine, clutching, and track/suspension to really FEEL a difference.