Honestly I've never bothered using a torque wrench on valve covers except on aircraft engines and rely on the "reasonable" approach. Tight enough to compress the gasket and prevent leakage but not gorilla tight to distort or cause the gasket to squish out.
I'm too lazy to look up what engine the HPX has but this document should have what you need. It's a bit old but covers the 3 & 4 T Yanmars used by Deere.