Hi Andreas,
you can get an approximate idea from this picture of a used crankcase for sale on Hitchcocks' web site. Apologies for the large picture, but if you scroll across it will give you more detail.
You can see one of the peg locating holes above the strap which holds the magdyno down, the other one is hidden by the strap. As far as I know the pegs simply screw into the threaded mounting holes on the underside of the magdyno, but I can't find them on Mr H's illustrated parts list.
Fitting a Bullet engine into a Crusader frame has been done for quite a few trials bikes, there is one on ebay in the UK at the moment.
This picture shows the magneto mounting a bit better, the Wipac magneto is probably off some stationary engine!
What you are proposing would be an interesting project, but if you want something light weight, you don't actually need a magneto.
A healthy Lucas RM21 or Sparx SPX005 alternator will happily provide enough power to run 12V ignition and lights using a Lucas 2MC capacitor or its modern equivalent instead of a battery. You can make the system more efficient using LED lighting too.
The other option would be a crank-mounted CDI ignition with some lighting coils such as the Electrex World systems:
http://www.electrexworld.co.uk/acatalog/STK-100D.htmlYou have a few options...
A.