I recently installed a Vevor F&R transmission on a Yerf Dog go cart a faced the same lack of pre-made parts that you are. I’ll give you a brief description of how I did it.
I fitted the transmission as close to the engine (clone 6.5hp) as I could and decided I needed a 8 1/4 output to input shaft distance. I raised the engine 2” above the swingarm. I used a 1/8” flat plate for the transmission mounting surface. From the parts that came with the transmission, I took what looks like an adjusting bracket and cut off the threaded adjustment(?) stud and used it as the forward transmission mount. I made two rear mounting brackets out of 1/8” strap. I bolted the brackets to the transmission, and making sure it sat level, I welded the brackets to the mounting plate.
I mounted the engine, and clamped the mounting plate with the transmission attached, to the swing arm. After much fitting work, removing the transmission mounting plate for grinding and welding, I finally had my 8 1/4” shaft center to center and the belt running true on the pulleys. I then welded the transmission mounting plate to the motor mount. The whole assembly now slides foe and aft to adjust the chain.
My torque converter is a knock off, and the transmission I sourced was supposed to match the driven pulley. I found it was very close but I had to disassemble the transmission and skim about .0025 off of the input shaft. Not a hard job but in my case necessary. I also cut the rear portion off of my torque converter so I could retain my belt cover.
I hope this helps.
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