Hey Chuck, I couldn't find information on the type of bike you're wanting to convert based on the info you provided nonetheless,

1st you will need to measure the bottom bracket size (or find that information on the web--but still it's better to measure it yourself) The bottom bracket for a bbs02 750 watt is (one size) 68mm to 73mm wide bottom bracket size. The BBSHD 1,000 watt standard sizes (you have choices) (68-73mm), wide and fat sizes (74-100mm) and extra fat (100-120mm). The newer Bafang torque sensing motors aren't even on the Bafang website yet but Johnny N.O. has the two of these "M-Series". Reach out to Johnny if you're interested in these (more expensive) motors. I can't find the bottom bracket size myself on line. Just be aware these torque sensing Bafangs are sort of in their beta-experimental stages, as of May 2024.

2nd, at least for the Bafang you will have to ditch the front derrailleur. I don't know of anyone who has anything other than a 1x system--that is one chainring up front. The TSDZ (Tong Sheng advertises you can have a double front chain ring--but I'm leary that this would be workable from my experience with my two Tong-Sheng builds their chain ring bumps out the chain-line pretty far away from your granny gears. But I've seen people on line say they've done it successfully.

Lastly, I'll just say 21 speeds for an e-bike seems to be really a lot of gearing to switch through while riding. As you've probably noticed while riding those 7 speeds with peddle assist, you can quickly get up to your top gear relatively fast compared to riding acoustic bikes.

Hope this helps, keep us posted on the forum on your build if you like.