Hi,
If you have the Sinclair battery top-plate and a working keyswitch assembly, then just buy a type 049 45Ah car battery, Ford Transit type battery terminals,relieve the battery post holes in the top-plate, and your away. I secure my battery to the loop in the rear bulkhead with a bungee.
NB this battery isn't as heavy as the original Sinclair battery, but being a starter battery rather than a cyclic type(the Sinclair battery is only a leisure battery so has more robust plates to resist heavy loads but otherwise is no better at surviving many long duty cycles than a starter battery) it won't do as many cycles, and it has less of range than a big leisure battery but it is cheap!