EPS _IS_ plain postscript. It is only wrapped by some comments and stripped
of any dubious commands for compatibility. You can simply do

%!

save gsave
/showpage {} def

% Include eps file here

grestore restore

showpage

% end of file

and this way show it on it's natural page position and size.

Programs may use the %%BoundingBox: comment in the EPS file to do
arbitrary scale, rotate and translate to include it in more complicated
ways than above.
