The Jett engine would be the way to go. That small case Jett 60 is the ultimate for the Jackal. I think that using the Jett tuned muffler is the way to go over a tuned pipe. One has to use a small case engine as the regular large case 60's are too large to fit. The motor mounts won't fit on the firewall otherwise. Oh yeah some of the Rossi engines are good candidates too, as are some of the MVVS engines. The OS 55AX wouldn't be a bad choice either.

I have entertained putting one of my Fox .50 engines on a Jackal. I happen to have one of the old Fox pne piece tuned pipes with the Flash Gordon styling. I also have considered using one of my old Fox Hawk .60 engines too.With 25% nitromethane fuel and a smaller prop the Hawk 60 is capable of 22,000 Rpms. But it would likely have to be a bigger prop to get the thrust up for takeoffs though. Unless you have a long paved runway. So RPMs on the Hawk would probably be down around 17,000 or so.

Retracts, I would suggest using what they suggest. I am not sure other retracts would fit OK or not. There is a long running thread over on that other forum where the members have been discussing the Jackal at length.