plenty of people run bike throttle bodies on zetecs, they can be had very cheap and then just choose your ecu and tuner and you'll be away. they usually couple the bike bodies to the st170 inlet manifold and engine, but you should buy one of these cheapish ebay jobs if you want it to fit the silvertop. you could probably do the whole setup for under £1k including manifold, bodies, ecu and loom if you're happy to do it yourself.