I am sure others will chime in here, but...I have a Raider with the same engine and problem. It had been doing this on-n-off for a few years, but always pumped up after a few seconds/minutes. Now it doesn't want to pressure up at all and just sits there and rattles on, even after I rev it up and let it fall back to idle, which used to work.

Getting to the oil pump? Ha ha, ALL you have to do is pull the cylinder head, oil pan and front cover and it's sitting in there behind where the water pump mounts. This is not very easy. To pull the oil pan (and some may say it's not entirely necessary), you'd have to raise the engine to allow the pan to clear crossmembers/differential. To remove the cylinder head ther's lots of stuff to remove and mark. Everything has to come off the front of the engine, too.

Deep breath? You bet. Lots of gaskets, too. Hopefully someone else can reassure you, but I guess it's not me...

John B.