It will run backwards with no problems, if you get the ignition setup properly.
If you want to stay with spark ignition, the easiest way would be to use electronic ignition from
www.ch-ignitions.com. The sensor mount can easily be put where you want it to run in either direction.
Another way would be to go with what we call "Gas/Glow". It uses a special fuel mix, and a glow plug and can easily run in either direction with no mods.
AV8TOR