I found my Tomahawk manual. I'll try and get it scanned this weekend.

You can do just about anything with the exhaust. There are a lot of machines around that have been changed so that the exhaust comes out through the engine cover, which works pretty well.

I'll see if I can find a big enough piece of HDPE to cover the hole in your machine where the original muffler was (the T'hawk will be HDPE plastic- they changed from ABS in 1975). How big is the hole?

Bigger axles are stronger, but it's up to you whether it's worth the expense and work or not. It is a lot of work, and all the sprockets and bearings need to be changed for the job as well.