Now defunct US company Aristocraft made a Class 66 in 1:29 scale in a number of liveries - Although well received there wasn't a big enough market to encourage further production from the initial run or models of other UK prototypes. It was designed to run on 45 mm gauge track to include the chunky code 332 rail based systems that most "G Scalers" use. Bachmann (aka Kader) now handle the Aristocraft brand and any possible re-releases from their old tooling. Might be worth contacting them to see if they have any future plans for this loco.
Std gauge prototypes are very much part of the garden railway "G Scale" scene here at GSC Jimmy - but mostly EU (LGB & Piko) and US (Aristocraft, USA Trains and MTH) outline. Admittedly UK outline std gauge product is almost exclusively G1 (1:32/10 mm scales on 45 mm gauge track) and tend to be seen (or see themselves

) as a separate branch of the hobby from the hotchpotch of mainstream garden railway "thereabouts" scales that happily coexist here

.
Out of production for a few years the Aristocraft Class 66's are somewhat sought after now and can fetch upto around £8/700 dependent on livery and condition. There were a couple for sale recently on the Garden Railway trading site on FB. Or check the usual UK dealers, a list of links to them is available on the G Scale Society's website, (and possibly German Ebay traders). Good luck with your hunt and welcome to this forum. Max
P.S. Unless your intended loco worked on the isle of Sodor I doubt it had any perception of its social class. No Gordon to look down on it for terms of reference