I can't see any reason why a more powerful pump couldn't be fitted..
However, the pressure guage would probably need upgrading. Also, if it has one of those variable speed jobbies then you need to check whether the new pump will draw more current than the old one. If it does then the speed controller may not be up to the job.
It'd make sense to check all the internal hoses carefully as they'd be more likely to split under the higher pressure.