For spare parts please try to contact MULTIPLEX directly (contact on MPX-homepage!). The MPX-forum is not the MPX-service and although MPX-staff members may read the one or other thread here it is not a very reliable way to get help in your case.
Your critisim about the LED linkage (only the two tiny legs) is true, especially if applying the gadget on a combustion-engine: I is too weak. I wonder whether it is possible to solder a new LED to some extension cable which would make application more convenient, even installing the main part of the sensor farther away from the engine.
In your case I would try to find a suitable LED at any electronic shop (your local dealer?) and try an extension lead as I suggestet above. I would be interested if you could find a solution!