I have a Lawn Genie L1034 auto/clock sprinkler valve on a pool autofill device. How can I change the valve to manual without replacing it? Can I replace the top eliminating the diaphram and solinoid? Can I remove the solinoid and plug it?

Sure... You remove the electric actuator and insert manual valve. The problem is, where you going to get that manual valve. I've never seen it being sold separately - only in set with anti-siphon valve. Check with your ACE hardware guy, he may have one in the back of the store laying around..