I compared a 7.5 and a 9.8 (1972 models), and the carb on the 9.8 had a bigger throat. I guess you could find a 9.8 carb somewhere and replace it. I'm not exactly sure that that is the "only" difference though. Most have different exhaust housings..these guys can tell you more about that (if you can there attention)!
The exhaust ports are a different size, that's how they can get different hp out of the same block.<br /><br />I have a 140 and the 115's specs are identical except for the exhause port size. Hope this helps, but be sure and check with a mechanic.