With the addition that the starter is actually having problems getting pass the TDC compression stroke at times I would say it is a compression relief problem. Plus considering it is a '71 model I say it is likely a worn camshaft then.
In order to replace the cam it would take one from a donor engine since Briggs no longer produces this camshaft. 211689 > 691167 > NLA
You ask if possible to add a decompression valve. I have never seen one done on these engines before but it might be possible. It would require modifying the head to where a decomp valve could be installed. Just don't what the maximum compression one of those valve from a chainsaw can handle. I do know some of them are exposed to around 150 psi. It would be interesting experiment.