I was routing about the stores and happened across a couple of universal tank sender units i bought a while ago and never used. Anyway opened up the box and they are neat and I think ok for the roadster tanks. Big scope with a 5" to 26" deep tank . These things were £15 complete with a 2" diameter guage from a marine/ebay shop.
Dont know about the ohms difference of this over the sierra requirements but seems like a fits all solution.
Edit. they have gone up a bit but still cheap
http://www.ebay.co.uk/itm/12V-FUEL-G...item564b01d682
Bob