I didn't even know Lucas made a 17AC ... are you sure it's not a 17ACR ? If so, then the Bosch unit from a 78 Ford Fiesta (with A/C) should work fine. Rated for 60 amps, plus puts out much more at idle than the Lucas units do. Bolts right up, connections are very similar if not identical (depends somewhat on which revision of ACR you have). And while not as common as some, should be on the shelf at any well-stocked parts store.
Was about $50 last time I picked one up, plus a $25 core. The units look so similar they probably would have taken the Lucas as a core, but I'd like to shop there again /bcforum/images/%%GRAEMLIN_URL%%/smile.gif