From the photo in the other post, it appears you have OS number 35900 FS-91 Surpass which was discontinued several years ago. Here's a link to the OS site to get a parts list for your engine.
http://www.osengines.com/parts/discontinued.html

Once you have the part numbers, type them in the "search" box at http://www.towerhobbies.com

You'll find that the rocker cover is $16.99 and in stock
The cover plate is $12.99 and in stock