I made some progress and received my new motor and some goodies from bms-battery (along with 40 euros of tax...)
I solved some issues, the pedals are now clearing the frame perfectly, the 51t chainring fits. But I found some new issues : I will need to make room for the left side chain, the tube is in the way right now, and the shock placement will be complicated..
I ordered a flux core MIG to weld this up, I found a model (made in italy) that does 35-95amp for 220€. I never welded flux core but I assume it is easier than stick, and it's still way cheaper than a decent GAz MIG. My tubes are 2mm thick so it should be quite easy.
The BPM motor looks great and well built, apart from the axle that looks plain ridiculous, i'm glad I have this one in a mid drive and not in-wheel
I fitted a 7speed freewheel with all cogs removed apart from the lowest one, 14t. If I keep the 51t, I will be able to keep up with the motor pretty nicely! But 51t looks a bit too big on this small frame :/
Overnight charger, not bad for 17$... if it does not blow up in the next 2 weeks.