A little diesel will only serve to be a good upper cylinder lubricant (valve stems). It would take a long time to foul the plugs and would not contaminate your crankcase oil whatsoever. Pure diesel will not be atomized by a carburetor the same as gasoline, but probably up to a 25% mixture would only produce excessive smoke like a 2-cycle engine. I would not even worry about your fuel filter since its job is to remove solids and trash. I can remember gasoline engines produced during WWII that could run pure kerosene.
I'm not old enough to have been around during WWII,
but several of those tractors and cars were still around when I was a kid and some still exist today.