I actually was just Googling this same subject about a week ago. You'll want to look for the "Bruce Guthrie Repower Kit". He has the machined plates that are necessary to close off the transmission areas of the old L's. I didn't dig into this deep enough to find his contact information as this is one of those "some day projects" for me. I have waaaay too many other things on my plate for the foreseeable future.
You might save yourself a bunch of time if you call Richard's Gravely in West Virginia. Most likely, they will either be able to point you straight to Bruce or they'll have the parts you need.
Remember - part of the deal of me giving you this great advice is that you have to post pictures of your project progress right here on this forum!! :thumbsup: