Add L.A.D. Specialties to your list of vendors to contact. They are excellent people to work with. Their engines are fast, reliable and repeatable performance, rebuild after rebuild. And their prices are reasonable. http://www.ladspec.com/contact.html
I’d add the cost of a full rebuild to the cost of the motor. This way you’ll know exactly what condition the motor is in. Pick a builder and stick with them. Check out the Yamaha fields before you decide on whether you’ll be running a pipe or a can. Your builder will like to know. Pipe engines are going to have more maintenance then cans. Lower revs with the can make the difference. Put a second Yamaha on your wish list. It’s nice to have a spare at the track and also have the ability to have one motor in the shop while you have one on the kart.
LAD Specialties / tony kart / rotax / kt100