I have a sneaking suspicion that the KSI guys used Lyta brand alloy tanks ( called Hi Point in the USA) but they may have been a special order just for those bikes. The son of the KSI guy has frequented this forum in the past so he may have the clues. Unless you want the bike to look as close to a KSI as possible, I reckon the Sportster tank looks pretty good, not my taste perhaps but it doesn't look out of place.