more floor and seat
If its drive train I would be looking at drive shaft or rear wheels..paint between the hub and axle, if using sloted mags, center with tapered nuts 1st (covered in old posts)
Swap the rear wheels over, if drums on the rear , still got the balance weights on them.
Engine not balanced!!!! not even pistons and rod weighed?
Any engine I have is fully dynamically balanced , even a daily...less wear, smoother ride, revs out quick.....
and you are going to drag it????
Very likely the vibration will be gone before the end of the weekend m8...and the sump. crank, rods
Or if not the engine, some part could be very defective..say a bent axle....throw you into a tumble or something.
When you starting hitting tracks throwing engines together with out basic balancing and prscision gets down right dangerous.