@DocRock is correct that you can use a field gage as military rifles usually run long and Forster's gages are fine. So are PTG's. This will never be a match rifle so minor variances in the chamber by headspace gage are not generally an issue. If you do not reload, then firing new brass is usually no problem. If you do reload, you have to pay scrupulous attention to the brass condition and usually neck size only for that particular rifle. The case web starts thinning excessively in a large chamber if you full length resize and then trim.
Alternatively, you can do as
@Slamfire recommends up above but I suggest you do a search and read his posts on that particular method--it does increase bolt thrust a bit but Slamfire has provided quite a bit of evidence that it doesn't do so excessively.
However, unlike IllinoisBurt, if your receiver lug locking surfaces are setback, then one of two things has happened. One is the case hardened surface of the receiver lug recess is worn through exposing the softer core that Mausers share in common. This is simply from the wear caused by the bolt lugs and receiver lug locking surfaces grinding each other or by some attempt at using valve or lapping compound to get the bolt lugs to a higher engagement percentage. This lapping method is often using in accurizing Mausers but it is critical to cut the hardened layer on the receiver lug locking surfaces to an absolute minimum. The case hardening on old Mausers can be very thin.
This can be fixed if you reheat treat the receiver which can only be done correctly at a few facilities. The difficulty arises is that Spanish Mauser, in particular, can vary a bit on their exact metallurgy which is why you need expert facilities doing this. Otherwise, the receiver can warp and that is that.
Another is to set back the barrel and do as IllinoisBurt suggests, however, if the locking surfaces of the receiver are worn through, expect that the rifle will go out of headspace relatively quickly which means that you have to more or less use a headspace gage to check frequently to determine if the locking surfaces of the receiver are set back again.
That method is really only useful if you plan on firing a couple of rounds to celebrate your grandfather and then put in the safe. Kind of expensive fix but people have different reasons for doing stuff. Another alternative is stick to low powered cast boolit loads to stretch out the life of the rifle.
The second type of lug setback is where an overpressure event occurred and the metal from the locking surface is deformed with ridges, valleys, etc. I suppose that someone could grind those off and then reheat treat the receiver or do something complicated like tig welding etc. and reheat treating but then you are at status one again.
This happens because Mausers, as a safety mechanism, are composed of a case hardened carbon steel skin and a softer inside. A Mauser that is case hardened through is brittle just as the 1903 brittle receivers are and these can shatter in an unpredictably fashion. So, a normal Mauser faced with a cartridge say of 65,000 psi is designed just like today's cars are--to deform the core while the skin keeps the material together instead of shattering. The only problem is if a person keeps firing 65,000 psi cartridges, the receiver lug shoulders keep deforming backwards in response to the bolt lugs being thrust back. Ideally, the bolt gets harder and harder to operate and extraction becomes difficult. These are warning signs that something is amiss. But, if Bubba keeps on with his experiment and the brass is stretched enough to separate, then hot gas from the cartridge is introduced into the receiver body with unpredictable results that just might result in a catastrophic failure of the rifle and/or medical consequences to the shooter.
My recommendation is if you find the receiver locking surfaces setback, then simply get another 93 Spanish receiver. Sarco is selling them for $20 and you can find quite a few on gunbroker, etc. for slightly higher prices.