Is it OK?
In one word, yes, but like so many other times, one word is very misleading.
It's OK but very difficult. If you do not have the tools and experience it is not likely to end happily. There are exceptions, of course, but in the vast majority of cases where amateurs have tried it they have either damaged the plug and/or the barrel or found that they could not get it back on in the same orientation that it was originally.
But you don't really want to remove the breech plug, you just want to be able to see what condition the breech is in. Unfortunately, that's very hard to do without a borescope. So, you compromise: you inspect what portions of the bore you can see with a bore light and presume that, if the indications are the previous owner took good care of the gun, the breech is going to be OK also. Big assumption, I know, but...
If you're really worried about the breech, or if the bore inspection is inconclusive, you might do this: take a wood dowel slightly smaller in diameter than the bore, round off one end to a hemispherical shape, cut a slot in that end and put a clean patch in the slot. Run the dowel down the bore to the breech and twist it around; remove the dowel and inspect the patch for crud.
If the gun has a patent breech the rounded end of the dowel should be sized to fit into that area, about .30" or a bit smaller.