For the seat, I regret that I do not know any supplier.
For the data plates, LWD Parts has at least some of the plates available.
https://www.lwdparts.com/product-cat...vrolet-canada/ I am always amazed at how well many European people speak English even if they have no need to do so in their daily life. Between Dutch, French and English I believe Frank and Stephan will communicate very well.
Brian Asbury (BCA on the forum) used to have blanks of the plate that lists the serial number of the vehicle and motor.
Forum member Luca Giovannetti offered 1 set of C15A plates for sale over a year ago, I don't know if he still has them.
http://www.mapleleafup.net/forums/sh...ighlight=plate