first i would pull the sump screen out from the base housing and see if it is stopped up, clean it up and reinstall.
secondly there is a strainer in the control valve that should be checked and replaced if clogged up.

I'm not sure on this particular model without a serial number but some of them had port screens between the control valve and the base mount (top of transmission), there were 2 of them that sometimes would clog up also.

also you might want to check the fwd/rev clutch and pump pressures