You might be able to fudge something yourself. I've had good luck searching eBay and just had a look. No Speed 300 motor plates, but there are some other goodies there to check out.
You could try Penguin R/C at
www.penguinrc.com and see if they still have a graphite motor plate in stock or could cut one for you. They do list motor plates for the stock 180 as well as Speed 300 and Speed 400 motors and dirt cheap at under $6 each.
P3508m180 - P3500/Micro 180 size Graphite Motor Plate
P3508m300 - P3500/Micro 300 size Graphite Motor Plate
P3508m400 - P3500/Micro 400 size Graphite Motor Plate