you can run most anything you want in Mod in SCCA Solo II (autocross), so long as it meets basic safety standards. Road racing is harder, there are a lot of fairly complex rules to navigate. as a tech inspector, i'm more than happy to help guide you through them.
your VW driveline might be useful in certain formula cars, or in a tube framed GT car. is it a 1600 (i'm assuming watercooled), or newer (and larger)? the early watercooled 1600s were used in the second generation Formula SuperVees, which are still classed as Formula Altantics, although they are rarely seen anymore. you could look at the newish Formula S which is for "specials" that don't fit naturally anywhere else. be aware that whatever you design has to pass muster in the club racing tech office in topeka; SCCA cars at places like Watkins Glen and Pocono get going fast enough that you really want an experienced engineer's eyeballs on the design at some point in time.
for a full fendered car, there are numerous spots where VW products are classed, although a rusty shell isn't an ideal starting point for a tub-based class (Production or IT).