you don't have to pull the engine, but I would pull the gas tank off, and give it a good cleaning before disassembly so that no dirt or anything can get in there.
I have pulled the jug off with the head still attached without pulling the tank or anything, but it'll make your life a lot easier.
put in a fresh exhaust donut while you're there as well.
I would buy both for extra insurance
40 bucks vs 200 bucks your choice»»»
Least you got to ride it with the toomey though