Those plates are not readily available unless Dave Degens (Dresda) makes them.
What you have to do is set the engine with transmission and primary in the frame with various blocks of wood in place to support them, and arrange them to:
(1) the closest reasonable spacing of the swingarm pivot point to the transmission mainshaft, and
(2) the lowest CG you can get that allows a reasonable routing path for the exhaust (if standard downswept pipes).
You can use stiff (coat hangar) wire to support the bits, as well.
Once everything looks good, carefully measure your critical mounting point dimensions and sketch them onto a clean sheet of butcher paper, then draft the potential outlines for the front and rear mounts. Cut out the mounts with scissors and test-fit them in place, adjust and re-sketch as required. Once they seem to fit, transfer the patterns to thin (1/4") plywood, and test fit. Once you are sure of the patterns, transfer them to 5/16" 6061 aluminum plate readily available at
Speedy Metals If you need any spacers, Speedy also sells 6061 alloy tube stock that you can cut to length.
If you don't have a band saw, you can cut this with a jigsaw or farm it out to a local tradesman. You can also do rough cuts with a metal cutting blade in a circular saw or angle grinder, then finish cuts with hacksaw or jigsaw. All edges can be filed them sanded smooth.
All of the hardware you'll need can be found at
Fastenal.
Order enough materials for two sets, and put the second set on e-bay to recoup your costs. You can also sell patterns...