First, do not try to rebuild your current turbo - Unless you have the shaft dynamically balanced, the turbo life will be very short. I recommend going with a quality turbo rebuilder like Turbo City for a replacement turbo. If you want, you can get an upgraded model with modified compressor and/or turbine wheels and housings for an additional $100 or so. They have one that provides about 30% more flow with very little added turbo lag.

Yes, you can substitute a Mitsubishi turbo (later 700 series used both mitsu and garrett turbos), but depending on which model it is, you will have to change the configuration of the turbo inlet and outlet housings to match the Volvo orientations, and you will need to use the appropriate 700 series manifold. Also, you may need to make a custom pipe for the top of your cat to mate up to the new manifold. If you use a manifold from an early 700 turbo, it will mate up fine, but if the Mitsu turbo is the later smaller version, you will need a 90- manifold or you will need to do some modification and machining to the early manifold to mate up with the turbo. Note that the later turbos also have a bypass valve where the 83 240 had none (relief valve in throttle butterfly instead). --
Steve Seekins