hi pal check the cable and gears inside the imrc are working smooth as if its jammed they self destruct themselfs then replace the tip121 there cheap off ebay its a 3 leg transistor inside the imrc as long as your ok with a soldering iron its very easy to fix this will only cost a couple of pounds so its worth a try and yes you can buy a used imrc and fit it yourself but how long a used one will last is a chance you will have to take but thats like any used parts really . i used the hightower link i posted above it helps you out if you have never done one before
hope this helps cheers mark