My understanding of the electronics is that the MC meters (which are even older than the MRs) used a Se cell, and those do go bad. The MRs use a CdS cell, which does not go bad, but does require a battery.
I recently had an external meter calibrated by QLM, and they charge around $80 for a meter overhaul, plus parts if needed. The cost may be the same for an MR, but it would be best to check directly with George Milton at QLM. If that cost holds, you might have good deal having the meter overhauled.
In terms of accuracy, my sense is that the MR is spot-on compared to my external meter, and I have always been happy with the exposures I set using it. As someone pointed out, you do need to be careful to switch from high to low-light range, but typically you leave it no one setting or the other for extended periods. Hard to make a mistake, as it will look dead if set on high in low light, and will peg if set at low in bright light.
Randy