I'm sorry for your loss! I dumped my Venus too and ordered a new one immediately. I use the stock muffler on the 120AX, mounted using the OS .90 template in the manual and the extension that comes with the engine (double check the angle so the muffler doesn't rub in the engine compartment). The vertical performance with this setup makes me laugh, every time! It's totally remarkable... Oh, got side tracked My point was that the muffler coming out the bottom looks totally fine, and is practically hidden behind the prop anyway.

Some great advice given to me was to run the engine in with the tank at the firewall. Once its broken in and the needles set, you can use a pump and move the tank back to the CG - the plane flies much nicer that way, and I'm eager to get back to flying the plane that way.