My wife has a Raleigh Select Six, yes that very first, very heavy but very nicely made original electric assist bike and she likes it. Anyway the battery is dead and having taken the cover off itโs an old Nicad pack. Can I replace this with a new lithium pack that would fit inside the casing and if so should I go with original 24v or go up to 36v and also what Ah should I aim to get? I have found various lithium packs online which will fit the case perfectly with some extra energy absorbing packing. Any advice would be much appreciated. Cheers.