I used to have a PT40 that ended up in a tree, It was a trike, but it had an air filter. Mostly because in the taller grass, I'd end up cutting the grass for them (without getting paid by the way, which is still a sore spot) and I'd find grass particles on the filter every now and then after those flights. They were worth every penny, but yeah, no one is really using them anymore.

Now I hear there's a whole new line of mixed gas engines that take regular unleaded at less than $4 a gallon with a drop or two of two cycle oil mixed in to keep everything nice and lubed. On the downside, they're VERY expensive as far as I've seen, but the flight times are outstanding. I've heard you can expect as long as an hour of flight time on a single pint sized tank, as compared to the guys at my club that get about 15 minutes of airtime before they run out of the pint of very expensive $25/gallon nitro fuel in the old style alcohol burners. I think if I spent 2K on a true gas engine for my plane I'd probably rig something up just to be safe. At that price an engine is an investment.

I love the smell, and sound of good gas engine, but you can buy a whole bunch of motors and speed controllers for that price.