there is a pretty steady stream of Zenoah g38's on eBay (mostly used, a few NIB). Used ones in good condition seem to go for about $70-90 inclusive of shipping, NIB as much as $200 inclusive. The G38 w/b (IMHO) an EXCELLENT engine for the GT DrI, would provide more than enough power (enough for STRONG aerobatics). If budget is the driving concern, I think I would drop back to one of the leaf blower engines in the 25cc range. There are some of those around (on eBay now) for about $25 each plus shipping. They are VERY easy to convert (some easy cutting/grinding), make a flat plate mount, add a prop hub and you are ready to go. For your purposes inside that huge cowl, you could use the stock muffler (gut it out to allow better flow) and not bother w/ trimming the flywheel as the vanes face aft anyway. You probably will want to swap the carb for one w/ 2 needles for better adjustment capability. IT will provide adequate power as the DrI is not a speed demon anyway, should be very scale.

Of course, the venerable Ryobi 31cc is always worthy of consideration. Plenty of them around still. Power isn't great but should be adequate for that application. Very easy to convert, plenty of conversion parts around (as well as converted engines). You might have to modify the firewall to allow for the length of the Ryobi as it has the carb on the back of the crankcase.