I was wondering if it is feasible to change this from working with a GamePort interface to working with a DB25 interface?
On PCs whose game ports support MIDI, there is a dedicated UART that is set to the MIDI data rate of 31.25 kbps.
In order to do MIDI on a regualar COM port, you would have to deal with two issues: the nonstandard data rate (for a COM port) and the electrical interface, which is RS-232 instead of the MIDI current loop interface.
Possible? yes Easy? no
If your DB25 connector is a serial port you have two problems.
One is that the data rate for midi doesn't match the standard data rates for a PC UART. So you either have to figure out how to run the UART at a non-standard rate or build a rate-converter.
The second is that there are no power pins on the PC serial port. This can be worked around by taking power from the data lines.
If your DB25 connector is a paralell port then you will almost certainly need some serial to paralell conversion circuitry and again the lack of power pins is an additional complication.
And of course you will need to write drivers that know how to talk to your adaptor and present it as a MIDI interface to the OS>