It depends on how much information is missing.
A Wallet Import Format key is encoded in base58. Therefore, each character in the key can be one of the 58 characters.
You mention in the comments that 21 characters are missing. Assuming you know exactly what 21 characters are missing (that is, you know that the first 21 are missing), you have 21 ^ 58, or
4.88336 ... × 10 ^ 76 Possibilities This is outside the range of brute force.
Also, if you do not know what 21 characters are missing, you have many, many more possibilities, since you not only need to go through the 58 possibilities for each of them, but you can also try all the possible locations within the key.
Unless you can find more information, the key can not be recovered.