The
myGurps site has a system for modifying the standard gurps magic system. Its by PK, who wrote it before he was hired by Steve Jackson games.
Gurps Thaumatology actually has a pretty impressive tool set to let you build magic spells and magic systems, though it doesn't look like that if you just start reading at the beginning.
And as roger said, we'd be happy to build something if you could describe the initial system to us.