In Germany you may have a gearbox to build en ebike from almost every producer if you ask kindly ( here in Germany there are alot of transmission producer).
I guess if you ask very kindly, almost any supplier would gain you a sample, if not for free when you have commertional interest. So it should not be too exhausting to get another of your 8:1 drives.
Btw suggest to keep distance betwenn BB and your motor box with a direct connection like i did in my (250W) tongxin project.
Chain tensioner for fools but better than nothing
flew away at 1,2 ridden meters but now i've got sth. better... just develop ...
(that's all the fun you'll get... besides of ... riding..?)