i ran into the same problem ,,so i ended up using 8 3/16 bolts and nuts ,,and made a adapter plate ,,so i could center the sprocket on the wheel ,, there was enough space between the hub and the spokes to dril 8 holes ,, it took time and you would need a lathe ,, but the simple way is to get the right back wheel in the first place ,, i now have a nice looking bike after 200 hours of work with a motor made in china ,chain problem ,air leak problem , clutch problem ,vibration problem,and today the spark quit ,my advice is buy a $50 bicycle if you are going to put on one of these motors ,, if you go first class with all the bells,,etc, buy a 4 cycle Honda motor etc with a pull start ,or wait for a couple years
there is going to be lots of changes and improvements i hope over the next couple years ,,i think i will wait before i build another one ,,100 hours is fun working on these kits ,,200 hours and still problem after problem no fun
japat100