I want to like the MVVS medium 2strokes too, especially the .91, which is based on the .60 case and very lightweight. However, after an experience with muffler bolts broken off inside the cylinder head, I'm very leery with any "bolt-on" style muffler. With a "bolt-thru" muffler, the worse is that the muffler bolts would break clean off and slide out of the engine block.

Anyway, as far as brands go, OS, TT, Magnum and Irvine all share the same bolt pattern I believe. TT and Super Tigre are both a little porky, so they aren't too good if weight is an issue for you. Plus the ABC version of the ST is not out yet. Otherwise, they are great engines.

Don't know if the Tomahawk is a 40 size plane or not. If it is, consider an Irvine .53 as a far lighter weight alternative to going with a .61 or bigger engine. At 18 Oz., the Irvine .53 can swing as big a prop as most .61 can and puts out darn near as much power, especially if you couple it with a tuned muffler like the Tower or Ultrathrust.