Two common mistakes...Plug leads on wrong cylinder and setting the timing after top dead centre. Always better to turn the engine forward to come up to the firing point, this removes backlash in the timing gears. Then it's a case of setting the maggy with points just opening, manual maggy set to full advance and assembling the drive gear without moving the magneto armature. With an auto unit wedge the ensemble fully open. More Tricks, Tips and Pitfalls in the Electrical Section.
Just make sure you have good compression and there are no mechanical issues. A good spark in the right place should fire cylinders primed with fuel or start yer bast**d. Problems keeping it running then point more to a carb fault.
Swarfy.