If it is used I would just walk away and buy a new motor for about $160, when you get a used motor you dont know how good they took care of it and how many gallons it has had ... and was it broken in right?
If iyour friend is selling it and he treats his rc car right and the motor doesn't have to much mileage then it MIGHT be a good deal.
If you want a better response please post more information about it.
I agree w/ dougc, we need more info. You can scoop up the Wasp .28 brand new for around $160. So, $100 for a used Wasp .26 doesn't sound like a killer deal. But give us a little more info on the engine use and we can help you out better.