The Mac II and IIx used one version of the PSU; the "real" IIfx used a revised version (with variable speed fans?). Part numbers are:
661-0375 Power Supply Mac II, IIx 120watt
66…
I have to agree, the PB1xx series are my favourite machine to work on, take the two case pieces apart and everything's right in front of you.
Speaking of replacing the LCD pane…