Offroad racing will, mostly because you fall over a lot, then you gotta pick up the bike again, mostly I reccomend it for the like minded-ness of bike riders. if you’re interested don’t feel the need for fast bikes, flat out at 50mph with friends is the best.
As for building mass, plenty of stuff online, but it boils down to heavy weights, short reps (5 or so) do that set about 3 times, focus on shoulders, back and legs, eat a ridcously amount of protein.