Yoshi's in bed with zuki and white-bros has traditionally spent their time with honda and now polaris.
Yamaha has GYTR, their factory performance parts. These are made by another company and stamped with yammi logos.
If you're looking for a good single:
Low/mid range: HMF
Mid/high range: Yoshi
If you want the best singles:
Sparks or CT
If you have the $$$ and want the power:
LTE duals.
If you're building a drag bike:
Barkers