Get an unbelievably small cotter pin or stiff wire to replace the little pin that goes through the metal cover and the top of the decomp valve before you start (easier then losing the little parts even though you put it all in a baggie). Also get some valve lapping compound.
Press down slightly on the sheep hook (where the cable sheath goes to) at the end of the decompressor and use sidecutter pliers to cut the little pin. Keep a little pressure on the sheep hook and remove the little pin. That should allow you to take out the decompressor valve and almost lose the little metal cover.
Use a 6 point socket to get the decompressor out of the stock head because she will put up a bit of a fight. Undo the hex base of the valve assembly, that holds down the spring as well as the hook that holds the bullet end of the cable. There is also a copper washer beneath all of that to act as a seal between the head and the assembly.
All the parts can be thrown into a plastic bowl and given a shot of oven cleaner and allowed to soak to remove the goopy carbon. Give it all a little scrub with some 3M scouring pad EXCEPT do not scratch the seal area of the valve itself.
Transfer the assembly to the new head, before you attach the valve stem back onto the sheep hook you will want to lap it into the valve seat of the new head. Use some lapping compound, you might need to beg some from a gear head who plays with 4 stroke anything because you are not going to need much. Otherwise get a pinch of kiln dried fine sand and make a gritty paste with a blob of general purpose grease. Put the Lapping compound onto the sealing surface of the valve and use a variable speed drill (at the slowest possible speed) grab the installed valve by the stem and rotate the valve and close it with slight pressure so the compound can machine the valve and the seat to a perfect match.
Wipe the lapping compound out of the head after you have lapped in the valve, that grit will mess up everything and could scratch the crap out of your new cylinder.
Attaching the sheep hook and metal cover to the end of the valve stem is a huge pain in the butt. You need to turn the valve so the hole is in the right direction and then push down on the sheep hook and install the cap then with your third hand put the pin in so it holds together.