I got a round acewell from here... http://www.electrosport.com/street/speedometers.php
Kinda pricey but nowhere near replacing the original from the dealer.
Wiring might drive you crazy. Hope you have the service manual, that helps.
You will almost certainly have to rig up a new way to hold the thing in place... make a new mount or modify your current one.
Sometimes there are Triumph speedos on ebay, maybe even a different model speedo would still fit your connector perfectly.
The Vapor by TrailTech is very popular with the streetfighter guys and a lot of the SV650 riders use it.
It is a good choice for the Triumph I believe because it incorporates a coolant temp gauge.
There are various mounting options and indicator light panels as 'accessory' options, but still very affordable.
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