I personally think that making an iron man suit would be the second coolest invention ever. Fortunatily, almost all the required tech already exists, exoskeleton suits and even plasma thrusters that use the surrounding air instead of fuel.
The most glaring missing element is the power source. As
far as we know arc reactors are completely fictional. So we are back to Obadiah Stane's problem.
At first I thought we would have to use a portable nuclear fission reactor like the one in the Ford Nucleon. But then I found out about super compressing materials using diamond anvils. You could super compress a substance into a small zippo shaped package that then expands slowly but with tremendous force. Attach screw gears to the top and sides of this "battery" which are attached to gear boxes and up gear the expansion force into fast and powerful rotational energy. Run these through simple generators (just motors with the + and - wires switched) and run the resulting power to your plasma thruster.
A bonus is that the expanding material would suck in large amounts of heat as it expands cooling everything around it so the battery might actually cool the generators and maybe the rest of the suit as well.
If the substance you compress absorbs heat too quickly and thus expands too quickly you might need to insulate it slightly to slow down its heat gain and consequently slow down the expansion. If you had a way to dynamically change its insulation you could control the level of power output!
So basically you use the diamond anvils in your awesome Tony Stark lab to super compress these "batteries" stick them in your suit and fly around until they are fully expanded. Then walk back to your lab and compress some more "batteries".
A slight problem I can foresee is that the batteries might be super heavy, I guess this depends on the substance you use and how much of it you use.
A more efficient although notably less awesome version would be to leave out the generators and swap the plasma thrusters for jet turbines that are axeled directly to the gear boxes.