r8's have no alpha, so no dice there. Although you can update old aircraft that use them to use bmp's instead, do this then an alpha might be possible. Ther was an app by someone to do this for planes, escapes me who, I used to do it by hand.
Another option would be to hex edit them out, tedious, 'needle-in-haystack' type code editing, usually with lots of trial and even more error. I did manage to do this with the Lago Hawaii (if you've seen the C-5's and F-15's you'll fully understand why.) but they were library objects and I hex edited out the calls for them in the main scenery. Older scenery is probably API, so not sure if possible that way.
The 3rd and possibly easiest option would be to ask one of our very talented resident scenery designers if they could do an updated version, lol.
:d
Jamie
PS. what scenery is it?