Re: Comet engine replacement
"I've also read that the left rear post will need to be removed to allow clearance for the muffler."
I do not know why this would be necessary using the regular Snapper exhaust/muffler, never heard of it and I have replaced plenty. Besides, the muffler will be on the right side.
The fender needs to be trimmed IF the engine has a bolt in the middle of the back of the crankcase to provide clearance for it. I usually just trimmed the entire fender off even with the top of the frame to match the newer model Snappers. A model 28---- engine will need it, a model 25---- engine will not. Both were used as 10 hp and you did not post the model number.