Where is the best place to get a rebuild kit for an FC motor (with hardened valve stem seals for conversion to unleaded)
It isn't the stem seals that are hardened, it is the valve seats. tetra ethyl lead was added to reduce the wear in the engine of valve seats and to inhibit detonation of the fuel when the compression was raised.
Fuels are quite good now with the removal of the lead, although some high compression engines still suffer a bit. All engines that have valve seats integral with the block / head require the fitment of hardened valve seats to inhibit the wear that they will encounter with the unleaded fuels. If operating on LPG, the same applies.
Keith