to put the D crank in a 504 you have to press the crank apart, and change the shaft that mounts the smaller bosch/motoplat flywheel and magneto set up for the 504. It's a huge pain in the ass. The process is as follows, assuming you have access to a good quality press, like a piston pin press at a machine shop. This is something that you really don't want to do unless you have the parts, time and are willing to make a rare part a super-rare part and them hammer on it for a day or so untill it lines up again.
You will need the 505 D crankshaft, and a 504 crankshaft.
an engine case that you don't care much about
a heavy brass hammer
Piston pin press
1. Press the connecting rod pin out of the flywheel side of the D crankshaft. The connecting rod and it's bearing can be changed at this point if needed.
2 Mark where the keyway needs to be on the D crankshaft half. Use the 504 crank as your reference. Use a chisel or punch to make the mark.
3 Press the connecting rod pin out of the 504 crank on the flywheel side.
4 Press the flywheel shaft out of both crankshaft halves.
5 Swap the 504 shaft into the 505D crankshaft half, making sure the keyway is lined up with the timing mark.
6 Press the crankshaft pin back to full height on the clutch side of the 505 crankshaft
7 press both 505 crankshaft pieces together.
Alignment. Al the shafts press flush with the crankshaft There needs to be enough space between the two crankshaft halves to allow the connecting rod to spin around with no resistance.
To make suer the 2 halves are put together straight, re mount the bearings on both sides of the crankshaft. Spin the crankshaft and watch for the clutch and flywheel shafts' movement . If one wobbles up and down, you have to use the heavy brass hammer to align them. It's a process of one hard hit to one side of the crankshaft, and checking, over and over untill there is no more wobble or run out. That's why a brass hammer- the hammer will take the brunt of any deformation, but will still allow you to move the crank pieces around the connecting rod shaft.
After doing this, and letting the parts sit for about a year, I put the engine together. It gave me a huge increase in compression, and was a little quicker off the line. The bad thing is, I've been thru 3 pistons since then. one circlip getting loose, and opening up the piston skirt. One set of broken rings that gouged the piston and the cylinder, and the current piston that is OK but with the cylinder in such bad shape, I need an overbore piston.