Generally you can reuse your existing 24VAC transformer. To double check, you can use a multimeter to measure the AC voltage (note: AC, not DC) of the output from the transformer. As long as it’s below 30VAC, it should be ok. Just plug in the output wires to the orange terminal block of OPSi, tighten the screws, and plug into OSPi board, and that should do it.
From your photo it’s hard to tell whether the space on the right would fit OSPi. It seems more feasible to leave the OSPi enclosure out, but even that it’s not clear to me whether it would fit. If you can measure the size of the empty space on the right I can tell you better.