In general, propane regulators are considered by their manufacturers to be "serviced as a unit," that is, non-repairable. It is for this reason that parts are not available.
In fact, many propane dealers will not, as a matter of policy, reinstall a used regulator -- on safety grounds -- even if it is working. Eventually the diaphragm fails leading to leakage, even if the gasket is sound.
In other than a museum-quality restoration where appearance is paramount it is my view that the regulator should be replaced.