I would imagine your main issue would be the input- MOS1 organs did not use a conventional -10 dBv unbalanced line-level input, and this would cause difficulties with any non-Allen amplifier. Later organs are different.
Every Allen speaker I've run into is 8 ohms, and I would image the HC10 to be no exception. Your organ is probably set up as 2 HC10's per channel on a 4-channel organ, presenting a likely 4-ohm load to each amplifier.
Because of the above, and the relatively low cost of a replacement S-100 from the used market, or a repair (the S-100 is a very simple topology than any decent electronics tech could fix), I'd suggest just repairing or swapping the amp. It'll cost less, and there are many less variables involved.