Currently, the best place to get conversion pieces is probably rc-monster.com. I am not aware of any other conversions at this time. RC Monster offers a specific motor plate for the 808, as well as a standard battery box that you can fit to the chassis pretty easily. This plate should bolt right in and not require any additional shims to match the stock XRAY 808 diff mount. Several people have used these pieces in their 808 buggies with great success.
Depending on your speedo of choice, you may need to come up with a mounting 'platform' for the speedo. Some have chosen to move the battery all the way forward and place the speedo on the chassis behind it. Others have kept the battery closer to the middle of the chassis and mounted the speedo above the rear driveshaft. Either way, the conversions do not reguire a lot of unique pieces. Just be prepared for ridicule and harassment from your local nitro racers!!
Hopefully that helps!
Bryan