You will need a Temp or Volt console gauge and a early VDO cockpit gauge. The back is the same as the 914 gauges. The new gauges are plastic with the same plug type BUT I do not know if they will interchange.
Remove the trim ring on both gauges being careful not to destroy the one on the 914 gauge. Your going to need it. Using a 8mm nut driver undo the nut. Looking at the back of the gauge you will notice the the plastic post is on the right. This is important in reinstalling the assembly. Remove both assemblies, being careful not to put greasy fingers on temp gauge face. Move the temp gauge over to the 60mm 914 case. Drop the center post in middle cross hole. Hold the gauge from the front and install back plug with the post to the right looking at it from the rear of the gauge. Install 3rd post and nut. Just snug up the assembly, you may need to move it around. Drop in 914 60mm trim ring followed by the 52mm trim ring. Center the smaller ring on gauge. Install glass and trim, making sure not to move inner trim rings off center.
Time to plug and test.