For practical purpose, and I think in the spirit of this discussion, is how fast a 50cc engine will spin a typical prop for the class.
I have quite a lot of engines in my database. 50cc +- 10% yields 21 engines.
A typical prop would be 22x8, so I queried for those data and sorted rpm descending.
50cc class, +- 10%
motorfabrikant motortype capacity rpm prop
MVVS 50 50 7650 22x8mej
DL DL50 50 7530 22x8 mej
DL DL50 50 7500 22x8 men
DL DL50 50 7350 22x8 xoar
DA 50-da 50 7200 22x8
GR-pro 45 45 7100 3W 22x8
Brison 52cc 3.2 52 7100 22x8
GR-pro 45 45 6900 3W 22x8
DL DL50 50 6800 22x8 JXF
GR-pro 45 45 6800 3W 22x8
DL DL50 50 6700 22x8 MSC
brilelli 46GT 46 6500 22x8 Xoar
CRRC Pro GF45i 45 6050 22x8 NX
CRRC Pro GF45i 45 5880 22x8 NX