I've done several weedie to glow conversions. To me as a sport flyer, about the only viable application is to give a 25-33cc weedie a fighting chance of powering a 1.20-1.50 size aerobat with good authority. With some careful selection, you can have a glow weedie come out weighing 40-45Oz with power close to OS 1.60FX territory at about half the price - all the while burning fuel that cost half as much as regular glow fuel, and producing 1/4 as much slime on your airplane.

BUT!!... The main problem is that weedies usually have very low CR and so don't make too much power on 0% nitro. Adding nitro back in increases the cost of the fuel and the rate of burn - kinda defeating the primary purpose of being frugal. Yes, you can run 4-5% oil if you use straight castor, but the weedies I played with (ryobi 31cc being one example) seem to make a bit more power with more oil content - possibly because it helps compression seal?

If you are willing to spend a little time tweaking these weedies for glow conversion, they'll make pretty decent sport engines.