Much is different, and I strongly suspect the economics are not in your favor. If you really think you would stick with the diesel for a long time, I think you should shop for a good one and then sell your 300E for the best you can get. Maybe the prices for it would rise in the spring or after the war is over (assuming we have one).

Obviously the engine is different. Glancing at a parts catalog gives some indication of what else would probably have to be changed. Keep in mind that you might not be able to get the diesel-specific parts used, in good condition, and would have to buy new ones.
The transmission is controlled differently so things like the control pressure cables are not same (although I think the basic unit is the same). The rear end ratio is different so you'd need a different differential. Some other drive line parts might differ, such as drive shafts or flex discs. If so,, the mounting might change. Fuel lines and fuel tank filters and breathers are not the same. Exhaust system is different. Cooling system looks to be quite different, including radiator and shroud. Some of heater hoses differ. The engine compartment arrangement is somewhat different, so even some of the things that could stay would have to be rearranged, meaning probable body work to mount them. For example, the windshield washer fluid bottle is a different size and shape and location! There are electrical differences, too, such as the whole diesel glow plug circuit, which affects items in dash, under dash, under hood, etc.
You can buy a lot of gas for what these changes would cost...
And believe me, I am a diesel fan and would have one myself right now if I could.

__________________
DavidB29
1992 300E with ASR
35 years of Diesels until now!