You can get a little more out of them. BUT, they don't respond to basic mods like the 2 stroke stuff does. YOu can't just grind out a hold in the leg and make 2 more hp. Just like a race car, you gotta look at it as a whole system. Open up the intake, then the exhaust becomes a bottleneck. Open the exhaust and intake, now the camshaft is going to be "too small". Change cam, your idle quality starts to disappear. Fix that with a pair of carbs, but the cyl head has to be changed.
IMO, leave it alone and enjoy it. I'm in your boat too...I'd love to get a couple more MPH out of it, and it eats at me all the time since I'm a hotrodder anyway. I did try a different intake (the part that's bolted to the carb). Original one on mine wrapped around the top of the carb, had to be a restriction. So I made my own out of part of an intake funnel from a 600cc crotch rocket. Tried it out, engine ran rich. Couldn't find anyone who sold jets for this carb, looked and called everyone I could think of, so I had to buy a few 112's and solder them, and then re-drill. Got the top end "right" and then had a bog in the middle. There is no needle to adjust the midrange, just a series of holes in the carb body. So without messing around with a few tiny holes, I just put a stock 112 back in and put the old intake back on. There was about 1 mph difference between them.
On another F25 I went about as far all-out as I could, knowing that I had a buyer for it even if it was loud and obnoxious. Ran the exhaust right through the cowling. Pair of stainless pipes, had to do some welding to the water passages, etc. Did some work on the intake, no air horn at all...and ran the engine on methanol instead of gasoline. From 29.5 stock to 34mph at 6000 RPM with no changes to the hull or anything else. I think if I had kept it and messed with the hull, trim, and motor heights, and propping, think I could have gotten closer to 40. I put my stocker back on and sold it to the boat racer....bunch of guys who get together once a year and race on a river in the eastern part of the state, for fun and bragging rights. Looks fun but not my cup of tea to be honest. I'd just as soon run 30mph, catch fish, and get back, without having to carry 10 gal of methanol, the noise, etc.