If you won't be using it as an organ but more of an experiment then does it matter if you blow it up? Also the output transistors are fused giving you some protection. I do agree that playing clipped audio at high levels will cause problems but you say you will keep the volume down. You are dealing with 1965 technology in a system designed for the living room, not a power head that goes to 11.
Wurlitzer didn't use custom transistors, but the specific parts might now be obsolete.