Hi Steve,
As you are probably aware that Graupner are quite long gone now you may struggle to find specific parts. I'm picking up useful remaining Graupner scraps for things when I see them for future use on a rainy day. As I found out myself recently on a rather expensive Graupner part, most suppliers state short stock or out of stock rather than it being discontinued parts (I wouldn't have bought the first one -and found it was missing parts from the box the supplier woudn't honour afterwards).
As you say you have the plans -any propshafts and propellers, couplings, motors etc. should be easy to be sourced to suit from usual suppliers, I recommend looking in Leeds Model Shop or Cornwall Model Boats first.
Depending on the part you may have to get creative and look for things that look similar. I'd imagine unless some rivet counter is looking you can find something close out there if not making yourself. Alternatively if you need a specific part, sometimes I've found obscure stocks for things serching for the exact part number.
For transfers / decals this might be a bit more tricky. I know BECC do allot of things, you may have to trawl the internet for something, otherwise if you can grab the images or details of them someone can maybe cut them on a vinyl cutter / plotter cutter.
Not sure if this helps,
Rich