These are on sale for $90, but the description says it's "Missing one dummy round from link".  
- Will this impact functionality?  
- Can I find a compatible dummy round to fix this?
https://armsofamerica.com/75rdak47drummagazine-sovieteratop-loadingoriginalromanianmilitarybattlefieldcondition.aspx

For functionality, it will load one more than its rated capacity (76 rounds), but it may not have enough spring tension to feed the last round reliably. The unreliable last round will take the place of the missing dummy round, but it will probably feed the first 75 just fine. Also, if not secured to the other dummy rounds, that 76th round will probably drop out when the magazine is removed from the rifle.

Find - maybe. Or make one if you're mildly handy with hand tools. The last dummy round usually has a beveled back end so the bolt will ride over it after the last live round has fed. You could probably make a workable dummy round from a fired case.