Your registration data is RSA decrypted, then checked for signature (DLP type). But the signature check uses the same modulus as in the RSA decryption part, giving you a clue to the existence of a subgroup, and the generator of that group. The exponent is capped at 7 bytes, making it possible to search cleverly within this range. Once the subgroup is found, a factor is revealed (and thus the other also). This is a long standing unsolved crackme on crackmes.de and unfortunately neotren had to clue us to the p-1 factoring algorithm, dropping the difficulty significantly.
Lately I read an article about how this one particular C64 user manual had nearly every detail of how the entire machine worked (insane depth compared to manuals of today) and then just a few days ago I read an announcement where the C64 is being re-released. I kind of want one of the classic machines for some reason :) And now this cool emulated game.