If it were me-
I'd use a Power Macintosh 8600, potentially with a G4 upgrade...but then I already have one. The advantage of that system is it does exactly what you want-it has an S-Video input.
As suggested above, though, since you have FW(assuming this is an MP 5,1-you should have 4x or so FW800 ports-I'm not where I can look at mine now but I think two on the front and at least two, maybe 3 or 4 on the back) you can probably find old hardware that will work. FW was made for this kind of stuff, and since it's a somewhat obsolete standard, often times Firewire peripherals now are inexpensive.
As a side note, I miss Firewire, and in fact when I dock my M1 MBP at my desk I even have a Firewire port on down the line(useful for some older externals). It was always a better standard than USB for a lot of reasons, but USB won for lower cost and being "good enough", plus USB 3.0 destroyed the last version of FW to make it to market in speed.