[villagetinker]

I am very mechanically inclined, but the references you attached did not provide enough info to answer your question. Ideally, you would need to get the physical dimensions of the existing and the proposed mufflers, including the mounting bolt pattern and gasket(?). With this info, you could determine of these are the same size (probably not), and take some measurements to see if there would be any clearance problems.

The other concern maybe with differences in the rate of flow through the proposed muffler. In general, the quitter the muffler, the low the flow rate, and the higher the back pressure. The higher back pressure could cause engine problems.

IMHO, I would call the manufacturer of your cart and ask them the question. If you have a cart that is under warranty, this will probably void the warranty. This is an interesting idea. One other thought, you could try searching Google to see if anyone else has attempted this, and see if they have any advice. I have in the past stumbled on to sites that had products for customizing golf carts, which would probably be a good source of information.
Good luck with your project, hope this helps.