Sorry, I should have been more clear, it's not the starter cover, it's the sump cover on the block.
 
Mod 20S232-1246F1

You simply have rust on the crankshaft preventing the cover from coming off.
 
Push the cover back on fully and with some sandpaper (maybe 120 grit), polish the shaft(especially close to the sump cover) until shiny steel.
 
It'll come right off.  A little lube on the shaft wouldn't hurt either.
 
 
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