Fit your new pump that you probably didn’t need. Check the timing cover crankshaft oil seal hasn’t inverted or split or been fitted backwards. The writing on the seal should be visible where not covered by the circlip.
Clean the oil reservoir mesh, check that all hoses are clear, get the 20W/50 oil level right, take the spark plugs out (people have to be told that, apparently), turn the engine over with the rear wheel in top gear or with the kickstart (in neutral!) and make sure that the feed and return sides of the pump both pump oil.
A clear hose pushed onto the scavenge pipe in the crankcase bottom should suck oil out of a jar.
Remove the oil pressure relief valve and you should see small pulses of oil appear in the cavity after a few turns of the crankshaft.
If all that appears to work, fit an oil pressure gauge and start the engine. If the gauge reading does not rise to at least 40 psi at 3500rpm, stop. If the engine knocks, stop.
If the pressure drops (much) below minimum levels as the engine warms up, you’ve probably got big end bearing failure.
To be sanguine, a 55,000 mile engine probably merits a look in the sludge trap anyway.