Some people have had good luck with a Zenoah G-20 in that airplane. Not sure how your 26 compares in weight. so is it technically doable? Probably. Just be aware that it could really affect wing loading and balance. Balance can be fixed but higher weight and wing loading aren't so easy to get around. It might fly but be a lot less fun and more difficult.

I'd compare the weight of the recommended glow engine for that plane to your 26cc gasser and decide. Don't forget to factor in the ignition unit, ignition battery and extra switch for the gasser.

Also, I'm sure you know this and it might not be an issue but since this is a beginners forum I'll put it out there. The Yak is a pretty advanced plane, definitely not something you want to start out on.