Bob,
I have 2 OS .91's with the big head. One is NIB the other was in my Byron F-16 and only lasted a handful of flights, (but I am a much better pilot now). The used one has about 10 flights and has a RPM rod installed which is a Titanium rod ($50)to make the engine blow proof. Tower wants 319.95 I will sell you both for $500 check or Pay pal.
Sparky