I think It looks straight forward to me , although the last time I dismantled a P1 was 1984 and the motor was used in a bike.
The spacers that I see look like they are sandwiched between the 2 frame rails and the motor base plate , that should be 8 spacers if you include the 2 used for holding the fuel tank and 4 for the engine bolts and 2 for the clutch .
If a couple are missing use washers unless there are a couple lugs welded to the brackets somewhere ,you will soon see when trying to bolt the plate down if it's sitting level.
