All the bits are replaceable. Raymarine can supply cups and/or vane and new bearings are available from a good bearing shop. I have done mine. Take one of the old bearings in to find a match or read the code with a magnifying glass.
just checked my file and the bearings on my ST50 are yours are probably identical:-
Raytheon Masthead Unit - ST50
The bearing code is DD 830 ZZ which means OD 8mm, ID 3mm, width 4mm and ZZ means non-contacting shield. Two bearings from NMB in Japan.