YO Scribbles, we seem to have a similar taste as I admire the Derringer bikes too. I'm working on my own version of something in a similar vein as the Derringer's. You've got a nice bike to add a motor to I must say.
Pretty much what I've read around this forum and MotorBicycling is, buy it from where ever you can get it the cheapest, as everyone's kits are pretty much the same.
I want to mention I haven't bought my kit yet and I'm not positive that everyone has exactly the same kit...I'm still feeling out the vendors and weighing my options still. The prices below are of some "80cc" kits being sold (which are actually 70cc).
-Thatsdax seems to have a good reputation as he advertises here and MotorBicycling forums. His 70cc kits run about $270 with shipping i think.
-Bob of BlueCollarBikes seems to be a good guy and offers his kit for $180-- around $200 shipped
then you have the Ebay vendors:
-BoyGoFast has kits up for auction all the time, it seems that a large majority have sourced their kits from them. I've seen thier 70cc kit sell for as little as $145 shipped.
-PowerKingShop claims to be the #1 seller on Ebay, they are preselling "80cc" kits due in on the 26th of June, they seem to be consistently selling for $180-$200 shipped.
-Livefastmotors is another big seller on Ebay. They sell a few different versions of the kit so thier prices vary.
All in all the median price seems to be around $200. I think it is worth spending the extra $40-$50 to have a vendor that has a good reputation and is easy to communicate with, which is why I'm leaning to vendors on the forum rather than Ebay.
Do some research and post here if you find any better deals out there!