I think I'd do it like so - keep in mind this is a preliminary idea:
Make the Power Armour a separate character. Give it all the usual aspects but make them pertain to the item's background. Make sure it has one or more 'Group/Party' Aspects. Be sure to give it aspects that revolve around getting hexed/stolen/hacked since these are primarily dealt with as compels anyways.
Give it Powers such as toughness, speed, etc... (as I mentionned above)
If you want to give it lots of boxes but lower armour, give it hulking size. This adds 2 boxes and makes it easier to hit. Give it lower toughness power.
Give it Living Dead. This means its stress track does not regenerate after a fight. You need to repair it. You need a workshop and tools and, possibly, fuel - see below.
Give it a Hunger track based on a character's drive (or other appropriate skill). This represents the user's ability to use the fuel efficiently. At the end of a scene, make a roll against the powers you used. Failure adds to the Hunger stress track. Consequences relate to damage and lack of fuel. It can be taken out this way until you track down more fuel (by missing a scene or by adventuring in-game). The nice thing about Hunger is, if you fail your roll, you can choose to take Hunger Stress, Take Consequences or Lose Powers to absorb the stress. Losing Powers reflects the degrading systems in the armour.
Give it some basic skills which represents what it can do on 'auto-pilot'. Like, if you call it to you or are controling it remotely. Basically, this requires one of the PCs to invoke an aspect on their character sheet "OWNER OF POWER ARMOUR" and the GM could just run it. Or, if your PC is really bad at athletics(for example), they can default to the AI skills.
Endurance +5,
Might +4
Alertness+3; Fists +3
athletics, +2 Guns +2
Drive +1 Stealth +1
If you want to drive it, here is what you do: Give it a pool of refresh for mimic abilities. This pool represents the 'drop of blood' you need to insert into the machine allowing you to use it. (also is compel fodder for anyone who steals it).
You use mimic abilities to mimic any skills/stunt/powers from the character who is playing/driving the power armour.
You can use the armour's abilities or your own, if you bought them using mimic, but All physical skills are modified by your DRIVE skill. So, you can use your own athletics, but your ability to dodge is hampered by your ability to pilot the machine. This can be done as a refresh rebate on the character. The AI is fairly simple and has a low Drive skill. All of its physical actions are also restricted by drive. The group can choose to 'upgrade' the AI systems at milestones by increasing the Drive skill. Eventually, they might upgrade it to a point they don't need the drive skill themselves because the AI takes care of most of the piloting. AI technology would be difficult to develop at a certain skill and would require appropriate research/workshop.
All mental/social skills(and Drive, if they want it to be higher) need to be bought using your mimic refresh.
So, each player plays their own character or the Power Armour character. Not both.
When I have some time, I'll write up an example sheet.