pardon this slight thread misdirection, but at least it relates...
the argument has been brought up before about motorcycle tires and other non-bicycle parts used on race bikes. some people think it's unfair, and/or that it's outside the definition of a "motorized bicycle," and a few other opinions that i can only describe as jealousy, either for not being able to afford them, or not thinking of it first.
the current tire of choice seems to be the Maxxis Hookworms. Maxxis is the new name for Cheng Shin. Cheng Shin has been making tires for over 30 years, makes motorcycle tires, bike tires, and specialty tires (wagons, trailers, etc...) they've been a popular brand of tires on many bikes, and odds are, you've had some on one of your bikes.
the hookworm tire, if you haven't seen it already, has a distinct wrap-around "tread," which consists of U-grooves on an otherwise slick surface. 20" ones come with a dual compound design, being softer at the sidewalls for more traction.
it's designed after motorcycle racing tires, and that's essentially what it is, only down-sized for bicycles.
bicycle design and related components have come a long way. most of the mountain bike design and technology is based on proven motorcycle designs. take a look at a high-dollar downhill mountain bike, and the only thing it really has in common with a beach cruiser is that they both have pedals.
but put a chinese two stroke in either, and they'll run in the same class. the rider's skill will decide the outcome.
as far as "fairness" goes, motorcycle parts are available to everyone who can afford them, same as every other bike, part, engine, etc, is.
if you're gonna bring "fairness" into it, then where do you draw the line? is it "fair" that one guy has a machine shop and can fabricate any custom part he wants, while someone else built their bike with a hammer and vice-grips?
is it "fair" that another guy has a ton of money and can buy whatever he wants, while his competition pumps gas for minimum wage and saved money for 6 months to finally buy a used junker and an engine kit?
is it "fair" that a master mechanic engine builder with unlimited access to state-of-the-art equipment can dial in a motor perfectly, gets to race against a guy with a dremel tool in his backyard?
"it's not fair" is most often used by the guy who lost.
racing is about skill, and about whatever advantage over the other guy you can get.
if we start placing restrictions on everything, we might as well just adapt the NASCAR rulebook, and make everyone have the same bike, the same engine, and just go around in circles all day long.
what's the point of having an "unlimited" class if you're gonna place limits on it?
look at the AGK bike that ran laps around everyone at willow. was it unfair that it had a motor on it that anyone coulda bought from harbor frieght? was it unfair that it had a highly-experienced rider on it? (some may call him a "ringer.")
the only thing i woulda thought was unfair about that bike and that rider, was if i didn't even get to see it because of restrictions, politics, or whatever.
so now, you're talking about an "exhibition" class, like only stuntmen (or clowns) can build and ride monster bikes.
the safety argument doesn't really fly either. it's racing. it's dangerous, we all have to sign waivers, and we all know what we're getting into. if someone builds a 20HP bike, or any bike that'll run in the unlimited class, i'm pretty sure they know the potential for disaster. anyone who builds or buys a high speed motored bike and doesn't think about the risks involved shouldn't be on the track (or the bike) in the first place.
the better the bike, the better the racing. things like motorcycle tires can make a bike safer, not just faster. if we can't use motorcycle parts, does that include safety gear as well? instead of buying some leathers for the next race, maybe i'll shave my legs, slip into some spandex, and get one of those awesome aero-helmets that are shaped like the Alien's head. i mean, after all, i'm still riding a bicycle, right?
my opinion, for what it's worth, is no matter what you build, as long as it has functioning pedals, it's still a motorized bike.