First thing to check, esp with FMJ mag, did you re-time the internal gears to the proper location.
I prefer to time a mag to the 30 BTDC method. I believe timing method for a WD would be the same as timing a WC and B. You will need help for this procedure
1. Remove the Mag and the Bracket as a unit. Remove the distributor cap from the mag.
2. Turn over slowly until you find the FIRE mark on the flywheel. the inspection port should be the one on the left side of the bell housing (when sitting on the tractor). If it is hard to see make a mark with paint, soapstone, etc.
3. With the #1 plug out, turn it over a few times until you are satisfied you are on the compression stroke, slowly start turning until the "FIRE" mark appears centered in the inspection port.
4. There should be one (or two) small raised portions, the width of the end of the rotor, on the inside of the mag. Rotate the mag backwards until the rotor is lined up with the appropriate one (if there is two, one is for 4 cylinder and the other is for 6 cylinder they should be marked with a 4 and 6). 5. Mesh the gear in the mag bracket with the camshaft so the rotor is close to the mark. Bolt the bracket down. Then loosen the mag and do the fine adjustment to line it up perfectly. 6. Replace the distributor cap. Your number 1 cylinder will be the upper left facing the mag. WC firing order should be 1-2-4-3 going clockwise around the mag.
7. If you were on the compression stroke and all the other pieces were timed properly it should fire right up.