Yeah, something like that should be easy enough. An MSR-24 with lots of different power buses, that's hard.
A quick image search seems to specify a PS-P2. As it happens, I have a PS-P2 handy and it says 12v, centre negative at 500mA. A linear supply (with transformer) might be your best bet.
Coincidentally, the first thing I found when searching for Tascam Porta 07, was a replacement power adaptor for £10 off ebay...