If you don't have two OS engines already (they are not made anymore?) you could instead get two of the Enya .15 engines. All (?) of their .15 engines have a rotatable front that allows you to run them in either direction, depending on how you mount the front part.
Pusher props are available from e.g. Graupner, but the selection is rather limited. I think they do 7x4, 8x6, 9x6 and so on, but I don't think they offer any 8x4 which might the most useful one on .15 engines.