The X30 would be a good bet right now. For 125cc TaG engines, it’s probably one of the more reliable ones, nice to drive, and it’s popular around the country so parts would be easy to get.
If you’re looking for something in between Yamaha and 125cc, you might want to look at the KA100 engine which is rapidly gaining popularity right now. Easy to use air-cooled 100cc, like a Yamaha, but far more reliable and has an on-board starter so it’s easy to go solo on practice days. And it’s great to drive.
The current price of a KA is good bargain thru the end of the month. Have raced a Rotax but not an X-30. There are more pluses for the X-30 over the Rotax. Must say the Rotax was very reliable. Keep in mind it needs a fully charged battery, the power value must be kept clean and the exhaust packing must be changed on occasion. Also to have it run as efficiently as possible you are going to need change the carb jetting and spark plug. Any decent priced Rotax may require many expensive modifications to make it race legal. Check any organization you might race with.
LAD Specialties customer / tony kart / rotax / kt100
The X30 is very strong and reliable, and you can still get great deals out there, many other older TAG packages may look attractive and cheap, but will cost you more in the long run.
Honestly the new KA100 will blow your mind versus the KT100, will last longer, will feel more powerful and more fun. You can still get some smoken deals out there, and does not need to be blue printed, just bolt on and go, and it will be fast out of the box, will probably compete with and older stock TAG 125 motor, or be pretty close to it.
I would buy either an X30 or KA100 period.
Or if you’re on a budget and just wanna have fun, you can just buy an LO206 and it would be pretty close to a KT100 with an can. Except no rebuild costs at all.