1fasthitman,
Look around at just about every forum and look for posts that mention engine size. There are posts everywhere that mention using engines that're appreciably larger than the largest suggested one for the airplane.

Reading those threads should give you a good idea just how much oversize you can go.

Also, another thing that'd be valuable for you would be to research engine displacement to weights. OS is presently marketing a newly designed 1.20 2-cycle as being "the size of your average .90 2-stroke". It's only about 5 ounces heavier than their 91. Why would you think they're doing that? And look for matchups like the OS 61FX--OS 91FX. They're basically the same weight. Actually only .7oz difference. There is actually less value to making an ultra light 91 for use on 90 size airplanes than there is making a 91 that'd balance up front on a 60-size airplane.