Earlier posts mention a "bag of bits"....
Reading back, a place to start would be the basics of the timing gears, crank, cam, idler gear and their static relationship. Firstly, are they all the correct gears as regards timing marks? Longstroke gears are identical in in form, ratio, etc, but the timing marks differ. More info in the Longstroke section, where this problem has been encountered, and the gears are illustrated so the difference is obvious.
Next, are all keyways/keys as they should be? All assembled correctly timing marks to spec, and finally a working mag set to fire at whatever BTDC? If the valve clearances are set, valve gear doing its job, oiled bore and sparks in the right time and place, a slug of gas and it should at least fire. If it doesn't, well that's me leaving stage left, as an aside, leave the mag end cap off to avoid any unintended kill switch fault.
Swarfy.