Hi Peter,
Good job on the tensioner and if you run out of skateboard wheels just get a couple of small bearings to fit on the bolt, don't bother with the sprocket.
The rest of what you have written is doing my head too much chopping and changing. If you have a carby that is working but have to put fuel down the carby to get it started that is either the choke is not fully closing or the primer is not priming. Coils , just extend the HT lead if it is too short.If the existing motor is running and not smokey then why think of replacing it? That is enough for me for now.
