FC540V works great in GP/MXR, and in domestic grass cutters & other commercial gas powered equipment.
I have 6 of them in racers now, all are over 30 years old and will still, yank you out of the seat. Bottom end grunt is extreme.
These work especially well for a "Heavier" driver
No Aftermarket parts because this engine is discontinued.
We are building one of these for performance use, everything is one-off and proprietary.