Electric bikes

You can get the bits individually, remember seeing drive unit/engines available on ebay.de, but only to domestic market addresses, you'd have to use a forward shipping company to get one. Battery and wiring parts are much easier.
My understanding is the European directive that covers e assist bikes means that unless you are a registered OE supplier/manufacturer and have some kind of traceability they don't want folk buying and installing kits themselves. Every proper e bike I've seen retailed in UK has a little sticker on it stating that it conforms.