My father used to race hillclimbs on a HD Sportster. Stripped of everything, frame drilled for lightness, etc. But still a hard-tail Sportster. Big ol' V-twin with 9.5-1 compression and kickstart.

Every now and then it would backfire and throw him over the handlebars. Once it tore the heel off his boot and caught him behind the knee and he limped for a month.

Good times.