With that much scum buildup, that engine will not have been looked after at all.
As UJ said, run a compression test, and if possible a leakdown test.
Then remove head and sump, inspect sump for any shiny deposits, and get shit dipped.
If there are shiny bits in the sump then your engine is due for a full rebuild / boat anchor conversion.
If it was me an engine like that would be getting a full rebuild. Or scrapped.
But that's just me. I've had enough "She'll be 'right" moments backfire on me in the past..