Sorry about the long delay, I finally got it "finished". I ended up not doing things up to the standards I originally wanted to, but I still really like the way it looks and runs. On the engine- I love the engine, starts right up, runs a long time on a little gas and is fine for my needs. It is however, underpowered, without question. If I put it in 3rd, I have to let out the clutch REALLY slow and even 2nd is a bit touchy. If I used it for anything more than running the wagon around the yard I would want a much bigger engine.
That is looking good. Those 6.5hp engines are a super buy. There is an outfit locally that has them on sale quite often. It's hard to fit them in though due to the different shape and the integrated fuel tank. Nice job on the repower. I'm sure it will serve you well.
No, the bigger the pulley on the engine, the more belt will be moved (faster with less power).
You probably won't want to go to small on the engine for fear of slippage (dogged belt will help with the small radius) and there will be physical limits to the larger size at the tranny, but a little bit on both ends can make a real difference. You'll loose some speed, but you're not racing it anyways.
If you look at the ratings on the Honda clones that are so popular you'll see the continuous rating isn't as impressive as the nominal HP rating. I had a look at the manual for a 13hp clone and the continuous rating was 9 hp. I think some of it comes down to displacement. A 14hp Kohler K321 is over 500 CC's while a 13hp clone is 390cc. Even a 15 hp clone is still a lot smaller then the older engine.