I would not get rid of any push-rods you have until you mock up the engine and check the contact pattern the rocker adjuster makes on the tip of the valve. The valves on the Manxman were a bit longer than they were on bikes made after a certain year, so if you don't know which valves you have then you don't know if you have the pushrods that will work for them or not.
I recently got all my extra pushrods together, about 40 of them, and measured them all on a surface plate with a dial indicator and they were all different.
A valve job or different brand valves, gasket thicknesses etc. will all potentially change your valve geometry. So just buying a set of early pushrods is going to be a folly unless you know everything else about your engine.
A lot of people surely just throw engines together with whatever parts, and very often they will get some use out of those engines, but they will not get the long life or power someone will get out of an engine in which they have left nothing to chance.
Good luck.
