I hope you like your Siren.
Hopefully others will chime in with their suggestions, but for starters, check my blog on the Siren. Lots of info therein.
Also check the Siren thread in this forum. A few guys who like direct drive are going with the MVVS "Redline." You can check it out at Esprit Models:
If you go much over 500W you'll want to put in a new firewall made from fiberglass or aircraft plywood (not lite ply).