You have to be able to build battery controller and display mounts out of aluminum angle, sheet foam, and stainless machine screws to put an aftermarket kit on most bikes. Use safety glasses when using power tools like a drill. I mount the controller under the seat out of the rain and the battery over the front tire so if it catches fire I do not roast the family jewels.
It is unlikely an aftermarket 500 w controller will fit where your original goes. 500 w controllers tend to come in an aluminum box 15 cm x 7 cm x 3 cm. If you go 48 v, the 36 v battery you have will be useless. Sell that bike operational.
It is more likely a bicycle out of the tip on garbage day would be more suitable than your carrera for 500 w 48 v service. If I wanted something folding & had pounds to burn, I would convert an airnimal joey. They are near Cambridge. Has 24" wheels, can be bought with big enough tires to not bruise your hips on potholes.
I've had 1300 w, 1000 w, and 350 w geared hub motors. First two were great but wore out. Identical replacements were not available. The 350 w motors were totally inadequate for carrying my 73 kg corpus and 27 kg groceries up the grades of my home county. Too slow, took forever to get up a 33 m 6% grade, and that slow slog burnt the winding out. I have later grades up to 12%. The motor burned before I got there. You have mountains in UK only in the north and west; consider your terrain when buying a kit.
