Although I have seen some raced, thats a very big, heavy chassis. Add to that the horizontal opposed twin engine, and the monster transaxle, its not of much use. Not saying you could not make one heck of a racer out of it, but not much in the way of parts for you there!
By the way, all v-twins are Over Head Valve engines, not flatheads. Opposed twins, like yours, are flat head engines.
Those do make great "Utility" mowers though, and many of us would love to have one for that purpose, so before you get too far, maybe you could swap it for a better racing platform! Where are you located, lol, I may just know of a guy with a pasture full of good racing chassis looking for one of those Big Brutes!