I get 300(+-10) when I measure the head with a digital thermometer. What should it be?

The only head temp readings I've ever seen are in Fly R/C magazine. The writer, Andrew Coholic, reports head temps and Exhaust Gas Temps.

If I remember correctly, those temps are in line with what he has reported.

There's no set head temps because of variations on engine design (area of cooling fins and so on) but ~300F is what I've seen on some of my engines fully peaked. I'm more used to seeing around the 240F mark because I never run them flat out. The coolest I've ever had at full throttle was 130F but that was running so rich (7000 revs) it was at the point of cutting out.

We once measured a perfectly-running O.S. Max .40 FP R/C engine at 345 degrees F when we used our contact probe at the airflow stagnation point at the rear of the cylinder head. We were running Omega 10% at the time, and the engine was leaned to just rich of peak RPM. The engine was used in our company trainer and ran for many, many hours over the years.