That transducer will work. You do not need to change the drive gear. The speedos are fully adjustable and programmable for any transducer.
Any 4" VDO speedo will work. The cheapest is the 120 mph model, model 437-155
http://www.egauges.com/vdo_mult.asp?Type=Speedometer&Series=Vision&Units=E
http://www.summitracing.com/parts/VDO-437155/
Probably cheapest in the USA. I found mine on eBay for around $70, but I looked for quite awhile.
edit: The kmh ones are REALLY expensive.... I gave up finding a cheap one and simply programmed the mph one to show kph. They are all EXACTLY the same physical units, just different faceplates.