Hey 4u -
The Sirio .18 is a nice engine that shouldn't roast your plastic Tranny gears and diffs but it's freaking expensive IMHO for a .18. Averaging $189.00 and then your looking @ $36.00+ if ya wanna get the kick ass Bibe bore pipe. Hell, you can get a mean big bore for less then $189.00 but then you have to spend $500+ to be able to run it...lol.
I'm not sure how the Wasp .18 compares to the XTM .18 but I'm pretty sure the Wasp .18 is producing 1.5HP and ported for low end like the larger Wasps. THe XTM might be a great engine for the price like Lykan is pointing out.