All depends on the engine size. If you are building for 049s then get reed valve Cox engines as they run in both directions.

The typical .40 to 1.20 engine sizes are well provided with 'pusher' props.
See http://www.apcprop.com/cgi-bin/store...c=6a&product=+

Above 120 it can be difficult to find appropriate pusher props although you can get wood ones custom made.

Some large petrols with reed valve induction will run in both directions!