From what I can find online it looks like the majority of the larger 48 volt lawn mowers run at the standard 3000 - 3500 RPM range that their gas engine equivalents run at.
Looking at the parts breakdowns of the motors I found if yours is the direct drive PM type I would expect you could run that up in the 5000 - 6000 RPM range without doing any damage whatsoever.
Also being designed for lawn cutting work I suspect that this type of motor has a considerable short term overload capacity as well.