Beins youuns an engr, get a replacement plug in preferably Platinum and ya won't be doin this again. One of the plug mfgr's probably has a cross ref. (I am too so I have the right to a lil fun.)
I like to run solid state ignition from Genessee and deep shank plugs like you find in current production auto's in my Fords that are/were gassers. (My Perkins 3/152 is Diesel so I don't know how Perkins gassers are set up.) I usually play around with some used ones (plugs) from autos that I kept in the shop and once I find a geometry and heat range that lets me idle without missing (at all) I go and buy a new set for the tractor.
If you do go to one with a long snoz, be sure and do an interference check with the piston first. I run the piston to TDC and measure the distance from the head so I don't run the risk of punching a hole in the piston. So far haven't had a problem.
I just think, no I know, a spark that sits in the combustion chamber is more effective than the OEM surface mount that essentially had the spark in the head.
My MF 35 has a lift up hood (from the rear). Makes things easier. Don't know if the 135 does also or not.