Hi Elliott,
The backtesting engine we use at Quantopian is called zipline, and it is open source: https://github.com/quantopian/zipline
If you have the forex data, you can write a new datasource in zipline and run a simulation on the cmd line. However, since forex data has a slightly different structure than equities, you will have to do a little more work to make sure the simulation behavior is valid.
hth,
fawce
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