Good luck with your bike. I am putting a Honda wt. a GruBee gearbox on the same bike.
I looked at your engine mount and had read a thread about the studs in the engine case vibrating and stripping out the threads. You might want to search the subject? Changing to American bolts and studs plus the proper thread lock seem to be in order.
I intend to change out all the bolts and studs that came with my kit.
I have not gotten to the upper tube and the gas tank fit yet. I am still grinding the adjustable mount for the engine to fit the large lower tube. Nice thing about the frame is, if you ever want to go to a 4 stroke engine, it has the room for it.
I searched a lot of bike shops looking for a frame to fit the engine and mount which wasn't too big for me. Jim