Depends on what it is that you want to monitor, your tinkering skills and how stock you want it too look. If you're not concerned with the fix being visible and aren't afraid to break out the toolbox it should be easily doable to fit an ordinary cyclocomputer to it.
You'd have to pull the covers off to expose the rotating parts, stick a magnet to the flywheel and find somewhere to mount the pickup.
It would require a bit of creativity, but should be doable with some double sided sticky tape, zip ties and whatnot.
A net search shows that your display has: "Scan, Time, Speed, Distance and Calories", which means that something like
this gadget would be a possible replacement. Have tried looking for a UK source, but there aren't that many cyclocomputers that have the calorie feature, probably due to the reasons below.
A word of warning: the numbers for speed and distance are basically arbitrarily generated for a stationary bike with no reliable connection to real life riding. A calorie counter that doesn't use a heart rate monitor is likewise rather tenuously connected to reality.
The budget version for you would be to simply ignore the broken monitor and merely track of what resistance setting you're riding at and for how long.
If it's the calories you're interested in, get yourself one of [ame="http://www.amazon.co.uk/Sigma-PC3-Heart-Rate-Monitor/dp/B00143UPNE/ref=sr_1_5?ie=UTF8&s=sports&qid=1247556128&sr=1-5"]these[/ame] (or a similar product) instead. Then pedal away for whatever target you've set yourself.
If you want to use the exercise bike to prepare for real life riding, get yourself a cyclocomputer with both heart rate monitoring and cadence monitoring, hook it up and start riding.
