If you had missed a few characters, you could find them through brute-force method. But there is no way to brute-force a private key with 35 missing characters.
No - I want to know the calculation of first 35 characters in wif (compress or uncompressed) format remaining 17 will be find with brute force.
The WIF and the private key hex are not entirely congruent which means that with a given set of hex characters, you will get slightly more WIF characters than you expected to influence (think eg. The first N base58 characters and part of the next one, so:
If you have M hex characters, then you should only count on the first
floor(M*58/16)
Base58 characters on being correct.