I have a Jett.50 and Jett.90. Both in prop jets so i know the engines fairly well. I also have two WM .P51s. In my humble opinion, the Jett.50 in that plane won't give you any better speed than a OS 50sx. I ran a G90 in mine and i got abopout 110mph.
I am building one for a friend right now with a OS61fx. he'll get a solid 95mph but remember we are at 5000ft so this is darn good. Gonna mount it sideways with a pitts underneath. One of the best smaller P-51s I ever flew. No bad habits and steady and smooth. I could let go of the sticks 5 ft over the runway full throttle and i wouldnt lose and inch.