I have a 66" hanger 9 P-51 with an VERY powerful O.S. 1.20 surpass. The only good thing about this big engine crammed,and I mean CRAMMED in it is that if your really screw up a take off(which is easy to do with a p-51),you've got the power to pull out of it.

The bad part about that big of an engine is that you run a big prop which makes it(at least in my case) VERY hard to slow up enough for landing unless you've got a big airfield. Not to mention P-51s are known for "nosing" all the time. And with an even bigger prop,your gonna break even more props.

So,stick with your 100......if your think later you need more power,then go bigger. I dont think you'd notice much difference anyway.