You might want to consider flushing out the fuel tank, pump and assuming it is twin SUs, the carb float chambers. Old Fuel gunge can cause all sorts of misfire problems and old fuel will have lost some of the more volatile components making starting difficult.
Replacement parts for SUs (and all new SUs made from modern alloys) are produced by Burlen Fuel Systems in Salisbury,
https://burlen.co.uk/ 
