I know there some law staying you cant fit bull bars/brush guard on more modern cars but can you retro fit brush guard to a 1998 FL1 if you have a Mantec sump guard fitted?]
If you can anyone know which ones fit best?

The bull bars mount up ok on a standard sump guard afaik the Mantec one mounts the same way so shouldn't interfere.

As regards the law on bull bars.... It is not illegal to fitt metal bull bars... It is now illegal to manufacture and sell bars that don't conform to eu legislation.....