How many miles are on the engine? First thing to do is remove the heads and look at the top of the pistons for damage. If you hammered it really hard you probably have misshapen a piston enough to score the piston walls. If so I'd ebay a entire engine and go that route. Otherwise, replace all the valves and valve seals. The guides should be OK. There are 2 types of head gaskets out there, the "old style" like your originals that stick to the head and the "new style" graphite that are supposed to give a little with the expansion of the heads during running. The graphite type is not supposed to require and sealant but I ended up replacing one after around 50K and put gopper gasket seal on the replacement. It was holding when I sold the bike about 60k later. REMEMBER the timing on the crank is a 720 degree thing!!!! You can have the flywheel set to TDC and be 36 degrees OFF!!!! When re-assembling turn it over by hand for multiple revolutionsusing the end nut on the stator.