Sid: Over on the Gunboards Mosin forum the general attitude is that mismatched bolts (and, obviously bolt-head- which is not numbered) are not a big deal with Mosins other than as regards value as a collectible. Out of curiosity I once got out the micrometer and measured the critical headspace determining dimension on a number of Mosin bolt-heads and was quite surprised at the lack of variability among them (in thousands of an inch). Soviet engineering standards were obviously quite high. The only one in my collection that was significantly off was visibly very well worn. If I was you I'd do some research to see if Polish specs are known to vary from the Soviet (unlikely) and, depending on what you find, source a good bolt body and a new-condition bolt head. Buy or borrow headspace gauges to verify the replacement parts are OK and your rifle will still have decent value.
milsurpo