British site doctordanger.com has a section on sprint racing & drag racing. Last year's Thundersprint Classic in Cheshire brought out 75,000 paying fans @ 8 GBP each. That's a million dollar+ gate. Includes RWYB (run what ya brung) classes for the masses.
Re: cash purses v. amateur racing: No one really has oversight, but, for example, I believe the SCTA is now a 501 C 3 "tax exempt" non-profit, which would limit their activities to amateur events with awards, but no monetary prizes. The question would be: Once you take a cash prize at BUB (in the LSR sport), are you still an amateur at BNI? Sure, professional race drivers come from other racing forms & participate. Nits to pick, true enough, but when the IRS audits a 501 C 3 non-profit, it's got to be amateur.
If Mohammed can't go to the FIM mountain . . . perhaps enjoining the AMA to act for the FIM in this country will accommodate the needs of some US LSR. FIM records at Nardo, Italy, in the big prototype "test bowl" are common. The FIM certified records at the Laredo Proving Grounds for Honda in '85 & 86. Maybe the answer is AMA LSR records, and a place to see them, keep track of them, publish them, whatever. Just like SCTA offers, only for all the other venues. But I would not want to be the "dart board" attempting homogenizing the rules. And if you want fans, get closer to them. All the LSR venues are arguably "remote."