neat design!!!
Perhaps you could use a bolt on section at the seat tube, running the upper added frame into the seat tube for stability and ease of modification. Also extend that section that runs over the battery to the headset and integrate a headset to that modified battery frame. The compression of the front neck of the headset, to the stem with a taller yet uncut front fork tube to the would stabilize that additional frame piece without needing it to be welded to the frame.

I currently have a heavily modified folding fat-tire e-bike with a 500w hub motor, as well as a fairly common RadRunner with a 750w hub motor. I think you would probably want a beefier hubmoter than topping out at a 750w, but thats just me.

Thanks so much, and I'll keep all of these in mind.

The reason why I want a 750w motor on the frame is because I kinda fancy putting an internal gear hub or a derraileur on the rear wheel, so it would be multi speed. Kinda like Dan Hanebrink's prototypes.

I just wish I can make this project a reality, but all I can do is draw. I still don't have a budget to build it.

Yaaa, hubmotor frees alot of space up to put towards bigger batteries however there is a conversion kit that uses the bottom bracket to mount a mid-drive motor and pedal kit. I know you are looking to non-pedals, but I think you'd probably have a better time in legal terms if you keep pedals available.

https://www.furaffinity.net/view/49977767/

Where I live, e-bikes must have pedals and a max motor capacity of 750w with a top speed of 20-24mph Class2 in order to ride any of the trails legally, but this sort of ruling varies from state to state. If it has pedals and falls into the above class it is legally considered a bicycle and shares legal rights to both road and trails basically giving it unlimited access to everywhere. My community is bicycle friendly so we have some abilities that other places don't necessarily have.

The bike that is pictured is a (very customized) Jank-E folding fat-tire bike. This thing folds up and I can put it in the back of my car!!! This is a 500w hub motor with a 32ah batterypack design, modified gearing with hills in mind. It tops out around 18-20mph but with a higher torque configuration vs speed. This configuration was designed around taking hills (mountain forest service roads)

Also, I would highly recommend the derailer vs that of an internal gear hub, the Derailer (depending on your model) will be FAR cheaper than a the hub you might be considering. Another advantage to the derailer setup is that it is an automatic chain-tensioner just by design.

As for cost, well, that above pictured bike started out at around $600 usd We added some massive double shoulder suspension forks and a suspension seat-post to even out the ride.
Not shown is the latest upgrade, a second battery rack setup. This summer there is going to be some serious upgrades issued to this bike, but over-all I think grand total it was maybe $900 tops thats including a dual battery setup. This started out as the lowest priced folding fat-tire ebike from ecotric, from a few years ago.

I was thinking of having pedals too as a variant. Since I kinda want to make this bike in reality. I tried searching for cranksets with freewheel on them but they're really hard to find. When it comes to mid-drive setups, they're really expensive. But the benefits of having mid-drives is you can change gears for speed and climbing hills without giving stress to the motor. Plus the center of gravity is on the middle.

Also, that bike is gorgeous.

Here, my design would be considered into the category L1B, where electric bikes (with or without pedals) should have a max speed of less than 50 kph. Also, motorcycle helmet is needed but license and registration is not required.

And I agree, internal gear hub can be really expensive. So I am thinking of just derrailleurs would just work fine.

I wish someday I could build this bike myself. I just don't have the money for it since most of my earnings will just go onto family financial problems and drama.

I don't know if you've seen this...
https://www.amazon.com/BAFANG-BBS02B-Display-Electric-Conversion/dp/B07NPN9RKN/ref=asc_df_B07NPN9RKN/?tag=hyprod-20&linkCode=df0&hvadid=334286824558&hvpos=&hvnetw=g&hvrand=13603293951197601398&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9033373&hvtargid=pla-680821220833&mcid=3058fbb3fa4c3ca38f742db81dd166d6&gclid=CjwKCAiA_5WvBhBAEiwAZtCU7wtkUybv0FQbch_NE6QOyS60eQINHAIupYTomy8njD71-RRSY74RMhoCmswQAvD_BwE&th=1&psc=1

But its a Bafang Mid-drive conversion kit, goes for around 470usd on Amazon if thats avabile for ya. I think its sorta what you are looking for.

I already saw it... It's just really expensive here locally. It's why I'm kinda looking for a budget build instead.

Amazing art :).