First check the oil pressure with a real guage, either electric or mechanical. Don't use the stock guage, even with the true guage mod. I'm a V8 guy so I'm not sure where a port for the guage is on the OHV block.
Then, make sure you have the correct oil weight. Its an easy mistake to make. Sometimes a clogged oil filter will drop oil pressure. Check the pressure right now, do a filter and oil change using a quality filter, and then check pressure again. If its bad then, you might have a problem.
Yes - the oil pump is in the oil pan, and the engine would need to be pulled. last resort!!