Hey Eric, welcome to the Forum. I'll give you my 2 cents worth, and you may find other opinions. My two favorite bikes to build are the 2014-2018 YZF250F and YZ450F. This has plenty of space for the battery, and the suspension can be really refined. My second choice is a 2018-2022 KTM 450SXF, and the KTM 250SXF of the same years is a little more work. The bottom frame tubes are 1/2" narrower, and make the chain lineup just a little more difficult. I have done a few Honda's but the space for battery is a little tighter with many models, and it is hard to sort all of them. I have built a lot of batteries, but Amorge now has batteries that I can use in those 2 bikes. If you build your own battery, you can use any frame out there. The YZ frames the battery goes in and out thru the top, and the KTM's thru the side, and both could be made for pretty quick changes. Most of the batteries we are using are 20S12P, I have a friend who had Amorge build him a 20S13P that fit in the YZ frame. You use the Molicel P42A cells, and you can have the current you need for motocross, or use the Samsung 50S cells for trail riding and more distance. Or use the Molicel P50B cells and get both for more money.
The QS138 70H V3 motor is a good choice with the Votol EM260S controller. If you want more potential power go with the EBMX X-9000 controller. ElectroandCompany.com sells the QS138 and EM260 package, or you can order straight China. If you want the more power, go with the EBMX X-9000 controller. ElectroandCompany.com will have a bigger Controller someday, but the Zapper Trumoto so far it hasn't materialized.
Lithium-king can definitely help you with mounts, sprockets and other stuff.