I had a close look at your decompressor, hard to see, but does it have 3 small clips holding the top on? (like in my image above)
If so you can strip and clean it, put it back together, test it by placing a tube on top and sucking on it to see if the valve closes.
If it is a sealed one, then you will need to get another one off a wreck or buy one.
I have seen a few of these with a spark plug blocking the decompessors port and the hose that used to go it, blocked off.
You could put an old spark plug in the port, block off the hose and try your engine to see if it works.
Should work provided you have fuel, air, spark, compression and good seals.
