James
Do mean they poured diesel into the cylinders in an attempt to loosen the pistons/rings, or do you mean they filled the gas tank?
The usual way of freeing up an engine is to remove the spark plugs and pour some kerosene into the cylinders. A good penetrating oil like pb blaster would work also. Let it sit for a few days and try to turn the engine by hand. Can you get a breaker bar and socket on a crankshaft or flywheel nut and try to turn the engine? Just go easy you don't want to break any thing. If it doesn't move, let it soak some more. If it does move, soak it some more and turn it by hand as often as possible before you try to start it. When you get it freed up, pour some motor oil in the cylinders and turn it over a few more times to coat the rings and cylinder walls before trying to start it. When everything is moving nicely and you have clean fresh oil and spark plugs, crank it and check for spark. Then add gas and see if the carb looks OK.