Old Lasco union check valve

I am trying to find a replacement insert or gasket for a check valve that is connected between our spa and pool. I have searched online and sent a request to Lasco, but with no success so far. The insert has o-ring face seals on both sides and a red rubber gasket that is supposed to prevent reverse flow. Unfortunately, half of the gasket is gone and so the spa drains into the pool within an hour of the filter pump turning off. It is installed below the water level and so water drains out - quickly - when I have it removed. So it would be a messy operation to cut the old valve out and replace it with a new one. I would greatly appreciate any ideas about where I could find a replacement insert.