Man, your options are nearly endless, depending on where you want to go. Are you looking for a drop-in replacement, or something smaller or something larger?
For myself, having just recently purchased a GOSM, I think that I would like a smaller burner for the things I will use the smoker for (beef jerky, salmon, and ribs.) If it were me standing in your shoes, I would buy a single burner camp stove, or even an electric hot plate, to replace my burner, but then again I smoke small quantities and tend to favor lower and slower cooking times. If you customarily smoke large quantities, or smoke at higher temps, your needs are much different, and you might want to consider adapting a turkey fryer burner. Or not. It's all up to you.
This doesn't even consider using your GOSM with charcoal...a guy could buy a stainless steel mixing bowl (or use the water pan from the GOSM, if he was so inclined) from the local supermart that would sit in the bottom and use the Minion Method (did I spell that right?) for the low and slow that we all crave...you might end up having to punch some holes in the pan for air flow, but only by trying it will you know...
View this as an exciting opportunity to rehabilitate or perhaps improve your GOSM and dial it in to what you need! Read, ask questions, read some more...or, just go for the drop-in replacement, if that is what suits your needs and ability. Again, it's your call.
The people around this place are more than willing to lend advice. All you have to do is ask!