The bolt head is the critical part in determining headspace with a Mosin. The dimensions relative to the locking lugs and the bolt face are critical.  But there is more to the system than a bolt head.  To adjust (change) headspace on the "end user" level here in the States, bolt heads are simply "swapped out". Every bolt head is not machined exactly the same and all of the recesses or channels cut for them in the receiver are not the same; fitting is required with newly-made weapons.  Fitting (or at least confirming proper headspace) is also done in the refurbishment process, since a variety of parts from different manufacturers are used.  Enfield rifles had the same process with three standardized "sizes" of bolt heads. Mosins bolt heads weren't specifically sized, but were fitted to a particular rifle. Different bolt heads can be exchanged to correct a headspace problem, but it is mostly by trial and error.  It is difficult to find a Mosin that will not pass a Milspec (not commercial SAAMI spec.) field headspace gage - even with mismatched parts.  The basic function of the bolt body in relation to the bolt head is simply to turn it to lock it and to unlock it. However, the interchangeability factor of Mosin bolt heads is great due to the cartridge headspacing at the rim. I would not mess with something that works. There is an old adage, "If something is not broken, fix it until it is". Way too many people are stuck on this "make it match" syndrome with arsenal stamps (or drawing and heat treat numbers on a Garand). That thinking is very dominant with collectors of American and German weapons and bleeds over into the Mosin community. The next owner probably will not care.

Afterthought:  A point to remember.  Since the bolt body carries the bolt's serial number, changing it out can or will result in a mismatched rifle.  That will do more to reduce the desireability and / or value of the weapon than a "wrong" non-matching arsenal stamping.  Most Mosin owners know that the bolt body is serialized after it is headspaced.  That is why most Mosin owners want matching bolts.  All of the small parts in a bolt - those that mate - also have to fit properly.  Changing one thing may necessitate that you alter other parts. 

It is up to the owner whether or not to impose a super-sensitivity to arsenal stampings or to leave something intact as it left the arsenal or refurbishment facility.

Another afterthought:  That big long lug that extends mostly forward from the bolt handle also has critical dimensions and important functions.  It is the "third" locking or "safety" lug in the Mosin system.  (Many folks do not realize this and Mauser folks gloat about the M98 action having a third locking lug.  Mosin implimented one 7 years before Mauser.)  Its rear portion is responsible for camming the bolt closed and its forward portion is responsible for camming the bolt open.  The lateral dimensions and spacings and clearances of the bolt head and bolt body combined are critical.  All the parts in the bolt assembly have to work correctly as a unit.  Replacing the bolt body could result in a bolt that will not close or a rear gap in the safety lug that is excessive.  Some things are deceptively simple, meaning they are more complicated than noticed at first glance. 

I think that one is asking for trouble and unnecessary complications by replacing good, servicable, working parts simply for aesthetic or cosmetic reasons.