if you just need 2D drawings, you can get pdf files at OS web site.
http://www.osengines.com/manuals/index.html
there are 3 view of all the engines they manufacture at the last pages of
each manual.
and those are pretty detailed and accurate drawins with dimensions.
you can convert vector drawings to dxf files using ghostview/ghostscript/pstoedit combination.
http://www.pstoedit.net/pstoedit/
http://www.cs.wisc.edu/~ghost/
all sharewares, not straigt forward process, but works.
if you want, I can convert couple of them for you.