As luck would have it, I just recently changed mine.
Have a look at the file: FlightGear/keyboard.xml
In there you will find defintions for various keys, defined by the decimal ASCII character code for that key. Space has decimal ASCII code 32 so find the block beginning with:
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<key n="32">
<name>Space</name>
<desc>PTT - Push To Talk (via FGCom)</desc>
etc.
Just change the "32" to the code for the key you want. For example, I wanted to use my TAB key for PTT since I don't use it for anything else in FG. So my change was to:
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<key n="9">
<name>TAB</name>
I don't think the "name" portion really matters but I changed it for future reference. Additionally, if you use a key that is already defined elsewhere in the keyboard.xml file, then you should comment out or remove the old block so that you're not trying to define the key twice.
Note that all of the built-in key help information will still say that Space is PTT. If you really want to change that, you'll have to modify the files that contain that information. I don't recall offhand which files that woudl be.
Hope that helps.
-- Mike