Do you mean comparable in weight or power? Well, G90 is not a power house for its displacement in stock form. Like tIANci said, it's only got the power of TT .91 4-stroke, which happens to weigh the same too. Coincidentally, both these engines like to rev higher with smaller props, so this is about as close as you get for a 4-stroke drop in.

I think YS 91 will be more powerful, and also swinging larger props, like 14x8 to 15x8. For Shoestring you want something like 13x8, and http://www.rcfaq.com/rpmstats/rpmbycc1.htm has a datum of TT .91 swinging APC 13x8 at 10800 rpm, at the bottom of this page.

You can get a TT .91 shipped from Tower for $199 - $30 w/ free shipping until the end of this month, quite a good deal I thought.

Saito 100/125 is going to be much lighter than ST G90 w/ muffler, so it will require some weight shuffling.