My chain was so close to the front of the housing on that tolerance I had to stick a socket around the drive gear nut and ratchet the gear around to draw the chain around with it. It barely cleared
If you mean the gear is so close to the inside wall of the housing (the wall where the spindle actually comes out of the case), there might be a missing washer or something. I dont know if these have a shim/ washer there or not. Im sure some motor expert here will know. If that is the case, you need a thin washer of some sort.
Also, if there was never a shim there originally, you need to figure out why that is that way. For example, pull off the clutch cover. Are the main drive gear and clutch gear in line or does the clutch gear protrude from the case as if the whole axle was shimmed to the right or something.
Hopefully your kit will have the parts diagram of all the pieces in the engine to show what (if anything) could have been flipped/deleted to cause this.
If all the parts are there, it could be a casting error on the wall itself or a casting error that isnt allowing one of the bearings to seat properly. Make sure the bearings are seated properly before you jury-rig something (add shims or die grind the casting imperfection)
I like the idea of examining the outer cover as well. That is an area that could easily have casting defects in it which could be ground off in seconds is necessary.
Maybe someone with more experience here will know right off hand.
Otherwise, congratulations on the purchase of your new chengine!