Hurt it Sept.'08 landing a moderate, maybe 8' high jump on my windsurf board with footstraps set too big. My foot, upon landing, jammed in too far and twisted hard inwards. I immediately knew something was wrong, and sailed in, very gingerly, no weight on that front foot.
Haven't run for a tennis ball until last week, when I ran for a lob to within 2' of the baseline. Everyone expressed concern.
I'd really worry about the power on a MX track. Think of doing doubles and an extra, unintended tweak of the throttle in the heat of battle.
I used to beat the BostromBros whenever we raced on a blue groove track, or on mud races. They'd kill me in wide open traction tracks (VetExpert). They rode CR-500's against my KX420 or RM-500's, which had much much less overall power, but more tractable and easy to ride. Even my HuskyWR-390, got some holeshots, just due to it's easy riding tractable power.
OTOH, maybe I"m a wimp at heart. Still, was weighing around 140 then.
I think you're gonna spend lots of $$$ on rear tires.