It’s good that you’re asking around. The more you ask, the more questions you’ll have. Can’t comment on either shop, but have purchased parts from Fastech and they have been very good to me. But do go the shops and ask questions. Ask what tracks or series they support. Which ever shop you choose, buy you whole package from one shop. They’ll be much more inclined to offer support.
As for being a newbie, I can’t recommend the Rotax. It’s a good and reliable engine. Parts are on the expensive side. No matter what engine you pick, You will have routine maintenance. If you have a strong Yamaha class in the area, I’d suggest the Kt100. But there is a benefit of the Rotax as it does have an on board starter. I have a Rotax that was a perfectly fine motor until the EVO mods came out. Don’t think I’ll ever get rid of it. A Yamaha with a can is reliable and less expensive to maintain. There is a lot you’ll have to learn about chassis setups. Once you’ve learned to drive smoothly, with fast and consistent lap times, then upgrade you engine
LAD Specialties / tony kart / rotax / kt100