I have four of these sturdy engines, that I use as test bench for mixes. What is the highest rpm you reached, and what propeller? They say 15.500 rpm but don'n mention the propeller. Maybe eliminating the induction has reduced greatly the speed for sake of flexibility?
A basic difference is in the upper crankase, where the liner fits in it. The oldest I owe has a flat crankase at this point. Other two, more recent ones, have the seats of the screws holding the cylinder that protrude from the crankase until touching the bottom of the aluminium cylinder. The fourth has a muffler, a very simple crown surrounding the exhausts, but the seats of the screws do not protrude until the cylinder. as the first mine. If you want to make a similar muffler the best way is to cut the protruding parts of the seats, if you have them, in this way the muffler is as simple as the original one and fits better in place.
Ugo