The newer midi addons are certainly a lot smaller! If you want to upgrade, there's enough people that would probably pay enough for the old midi interface so sell that old clunker i/f on Ebay and put that towards a newer cleaner one, for example: CHD ELEKTROSERVIS
At the very least, seperate out the midi connector wiring - remove them temporarily and get them away from the rest of the circuits (without shorting anything to the related parts), temporarily elevate the midi board away from the board and see if that helps.... worst comes to worse, you can install a metal shield between it and the board beneath, but you have to ground it back to the supply and make sure it doesn't short out anywhere... some of those chips can get warm, you certainly don't want a chunk of styrofoam there either, firstly as it's a static generator that can zap your chips in the right circumstances (more a theoretical, but still possible) and it will cause heat build up of the 'insulated parts'... premature failure if it's bad enough.