Found directions for how to take the check valve off and service it. Did so and it is currently undergoing leakdown testing as I write (sounds fancy, actually all I did was prime it and hang it upside down to see if it holds oil like it is supposed to do.)
Used a .177 caliber cleaning brush to hone the barrel of the check valve to make sure the spring wasn't hanging up on any crud on the inside.
Just to give me another option, I ordered the Kawasaki version of the part and should have it on Friday. So far, so good.
Also ordered a rubber dam (which I had previously thought unobtanium) that is to be installed in the engine case to allow it to hold 1400cc's of oil in the transmission housing area. In the early bikes, like mine, the capacity was 1200cc's and there issues with gears four and five wearing prematurely due to oil escaping away from the transmission area. So, assuming the bike runs okay on initial setup, I'll be ready to do a more complete tear down in the fall/winter.