If you go for the 118 you will be put in the modified class. If it's a fully worked engine, it'll be in the unlimited class. Unlimited has a 160cc Honda engine throwing down 20+ hp. Get in touch with Neil if you plan on racing the 118cc lifan. If it shifts from an engine transmission, you need to pick one gear and use it for the entire race. I can shift (for now) because my hub shifts, it's happening on the bicycle side.
All I'm saying, is if you plan on racing, make sure it's legal for the class you plan on running it in. Try to bring all the safety gear required, you might be able to rent, or borrow gear.
Neil can confirm, don't take this at gospel, but I believe anyone can show up and pay just for practice day and also get to play during morning practice before the races and during lunch break. I might be wrong, confirm with Neil before attempting.