Considering the complexity of a wallet private key, the odds of guessing a 8-characters alphanumeric private keys are 1 in 2.1834E+14 (62^8). That's one in 218 trillion-plus. I don't think you have the chances of restoring those letters anymore.
Actually ethereum private keys are 64 Hex characters.
@op, if you don't know the exact positions of the missing characters, then your chances to guess your private key are practically near Zero.
If you know their positions and assuming you know the corresponding address then you can use any brute-forcing script that will try each possibility, derive its corresponding address and compare it against yours (there are 16^8 = 4294967296 possibilities)