The engine in your Excelsior is a standard 125cc 9D Villiers. The top half of this engine could be assembled in various ways to look a little different from all the other makers using this engine. It looks different but remains absolutely standard.
Transfers are available from either Classic Transfers or the English Vintage Motor Cycle Club who can also supply copies of catalogues, handbooks,etc. Often you will find the handbook for prewar lightweights like this aren't very helpful and in that case you get yourself a separate Villiers handbook for the 9D engine. The VMCC can provide that also.
The 9D unit was used by many different manufacturers and are quite easy to find parts for. This is not a fast bike. 40mph (65 kph) is about it, but they are very cheap to run. Unfortunately they are also a bodgers delight which doesn't help their reliability. Play it straight and it will keep you happy for a long time.
Good luck, Bob