Hi Bob. Either engine should work fine. I will say, one of the later Orwicks have a slight power advantage over the Spitfire, and the timers are better in my opinion. I have flown my Taurus with both of them, and the Orwick with the timer advanced will pull your arm off running in a four stroke.
I added an extra head gasket to mine because they come with 10 to 1 compression. It would be fairly easy to make it interchangeable so you can run either engine. I think I have had at least three different engines in my Taurus. Both the Spitfire, and the Orwick, plus an Atwood Super Champion. It actually started out with a Fox 59 short shaft.
I have no experience with the Forster 99. I am sure it would work well also. The 29 Forster rear valve ball bearing motor I have is very strong.
So, I hope this helps. It should be a head turner for sure. Wear your best track shoes. LOL. Maybe tie yourself to a stake in the ground.