things are rarely as simple as plug n play. here we preach to avoid premade ebikes and hub motors like the plague, go with a mid drive motor kit if anything, the torque and performance, ease of install and maintenance puts hub motor setups to shame. as far as swapping wheel sizes on a bike frame, generally you cant. a frame is usually meant to house one diameter size of tire. the tire width is something you can change generally within a range. you could measure your frame see if it has another 2.5 inches of clearance at least around the front fork and rear to see if it can fit a different size, but i doubt it. so back to our main advice, find a good mtb hardtail frame that suits you and your use case, then look into converting with a mid drive kit. see johnnys many videos on conversions, you will learn a lot from just that. and the bafang install guide. you can DIY easily