Don't know of any snap-caps especially made for rimfire firearms. They do sell "training rounds", but these are solid aluminum, and would be just as harmful to keep dropping the firing pin on as the gun's own structure is.
One possible solution is the use of no. 6 (yellow) drywall anchors. They do get flattened at the rim very quickly, so you have to rotate it in the chamber a few times, then replace it with another.
The issue with dry-firing your M63 is the same as with any rimfire gun; the firing pin smacks against the inner edge of the chamber which normally supports the rim of the cartridge. This impact, on a repeated level, can result in peening (flattening) of the sharp edge of the firing pin, and also in depression of that chamber edge. Either of those two conditions, or a combination of both, can cause poor ignition reliability in the firearm.