Two cents worth
My buddy has a GT as well, he's been running a SH .28 all year long. It's a fantastic engine, has all the power you'll ever need or want, easy to tune, doesn't overheat, sips the fuel (typically eight minutes on the track to a tank or a little more) AND....got it for 87 bucks new on Ebay. Comes with carb and you can typically find it either for bump box or with pull start. It is the best 28 I have ever seen. Plus, you can get the plugs for it for about three bucks a pop online. This is one of the few things around that you won't be trading off on losing power for cost, because I've seen plenty of engines that cost a lot more that don't have the power this thing does.