Sorry, this is later than the other posts above so has been overtaken by them but I include it for completeness.
Fortunately those AC regulator modules are still available from Lionel (and now I note that you have one already).
This is unscientific and I don’t want to confuse the issue, but I recall a thread a couple of years back where we were discussing this generation of Lionel smoke units and this PCB was mentioned, which is still available from Lionel but you have to search under replacement part number (not product number) for 691SSMUK02:
The part number printed on the part, which I have got from Lionel in the past and used on one of these units, matches your photo above. But replacing the PCB won’t restore operation if it’s the AC regulator that has gone.
