I'm new to the forum, but I've recently been getting into N-scale building with Metcalfe kits and ScaleScenes self print models, and I've been really impressed with the level of realism that can be achieved.
I've been wondering if card rolling stock could be a viable option. For one thing, respraying UK stock models tends to produce less than satisfactory results, and destroys the resale value of the "donor" models. However, card models could allow the chassis and bogies of off-the-shelf models with card uppers would allow the donor model parts to be stored for later reassembly if resale required.
The 141 is a very challenging shape to start with, and I think you've done a fantastic job. I actually like the uncoloured version, and I'd encourage offering that as an option. I think pairing it with the Kato chassis is a great idea, as they seem to be available cheaply on Amazon.
Offering them pre-cut and scored would be fantastic, but if that is proving problematic, would offering a self-print kit be a less stressful way of making it available?
Other things that this approach might work well for would be DARTs (I think you've already done most of the work for this at OO scale) and Irish coaching stock.