The serial check looks straightforward. First the serial is rearranged and parts are changed into integers. Next a hash is made of the rearranged serial. Then a division with a magic number at 489620 is performed and the result determines if the serial is valid.

Since you have a valid serial, you can use a debugger to step thru the code and see the results of each instruction. Then use a bad serial to see how the results differ.

From the two different results you should be able to see how to reverse the procedure and create a keygen.