I took a look at the crossover I have in the old parts stash, and it looks like that fitting may be a British standard thread. The closest standard would be 1/4″ BSPP, assuming it is a parallel thread. It looks like it as near as I can eyeball it. 1/4″ BSPP is 0.5 in OD with 19TPI. My thread gauge skips from 18 to 20 TPI, but I think this might be a good place to start. Discount Hydraulic Hose (https://www.discounthydraulichose.com) sells BSPP fittings.
I found that the fitting on the oil gauge steel line mounted to the side of the engine also uses a BSPP fitting, in that case 1/8″.