I am rebuilding an Xr400r, and have a battery pack that I will be fitting.
It is a rechargeable AA pack, and it fits nicely in the bottom of the airbox, still with kickstart.
It should be capable of supplying enough current for fuel pump priming until the engine is running.
You should be able to something similar, not sure if a normal capacitor would be enough, as it will slowly discharge when not in use, but then again, it might work, it just might take longer to start.
Don't think that the fuel pump has been decided on yet.