Hi

Got a 2005 Rev 3 250 needs new rear master cylinder

The aprilia rs125s of that era had grimeca master cylinders that look identical with 40mm bolt centres

Any reason why it shouldn't work if I fitted one

There are brand new pattern ones on eBay for 35quid including reservoir and hose compared to 90 for a grimeca

Thanks

https://www.ebay.co.uk/itm/112023555182

No reason whatsoever, I did similar with my old Rev 3. Only used a Brembo one

Any master cylinder will work with any caliper, it's only a simple hydraulic system. 

The thing to look for is the bore size of the master cylinder. Changing the bore size will change the power and feel of the system.