It's possible to use a repro battery box which is very close to the original battery cases used on WD motorcycles. These boxes provide enough space to accomodate a modern battery. That is in my opinion the only option to have your battery to look like as much as the original.
There is a whole range of modern batteries which are suitable to use on your M20, the dimensions of the repro battery boxes limit what can actually be fitted.
The repro battery boxes are made by Arnaud Leger, if you search his name on this forum you will find his contactdetails and some additional information.
