It would be a good idea to fit one because it'll make it run lean and hot if the decompressor is able to open on the induction stroke and draw air in. My decompressor cable got trapped under the tank once and was being held open ever so slightly. It melted the rubber anti-vibration dampeners I'd fitted to ther barrel!
They can be blanked off if you want. You could even blank it off by screwing a sparkplug in there if you don't want to buy a proper blanking plug. Depends how difficult it is to get over compression on the kickstart (I find the clutch slips on mine preventing it going over compression if I don't use the decompressor)
Otherwise re-instate it. You'd need a cable, lever, spring and probably the little bottle-cap looking thing that goes on top of the spring.
If you don't want to set it all up now, if you're going to run the engine at least get the spring and bottle cap thing so it holds itself closed. That'll set you back a whopping £3.98 plus postage. Parts 140100 and 140101.
DO NOT PULL THE SPLIT PIN OUT. If you do the valve will drop into your engine. Don't ask me how I know.