Makes sense, water is heavier than oil and they don't mix, besides it sat for days.

IMO, the only safe fix is. Remove the engine, completely disassemble it. Crankshaft, pistons, cam, rockers, everything. Flush with water, dry, reassemble with new rings. You've got to get all that sand out of the engine, else its death.

Keep track of the part order, so you can put all the pieces in their proper place.

Oh, a water pik would be a useful tool for flushing the small parts and ports.

Good winter project, hope you have a nice warm & dry place to work.