Why don't you rebuild the class 14 back to original, with the proper Paxman engine?
This is an interesting moral question that stretches far beyond the simplicity of the question and reflects the background experience of Andrew and his father.
There are some locos in the collection – such as “Grace” (last s.g. Loco built by Hudswell Clarke) and "Coronation" the first diesel hydraulic built by NB and the oldest s.g DH in the UK - which Andrew regards as having historic significance and will not therefore be materially changed.
However, for others the idea of returning a loco to "original" presumes that the original condition is the only form in which the locomotive could exist, or that it in some way is the only "ideal" condition. The history of locomotion is littered with designs that were not right first time and modified and improved as new ideas, technologies or components became available.
The Class 14 901 (originally D9524) had already been repowered once in industry (its current Rolls-Royce is its third prime mover) and returning it to “original” is neither practically nor financially worthwhile. (It would mean sourcing an obsolete engine, rewiring the locomotive to 110V, new exhaust and air intake systems, re-made plumbing and opening up the Voith to de-gear it back from 1800 to 1500rpm - all of which would just make it "the same" as all the other Class 14s in preservation.) Similarly Andrew always took the view that any further Class 14 that came his way would be considered on its merits - if complete or nearly so, then the Paxman and 110V systems would be retained. When he was offered D9500 in summer 2010 he thought long and hard. The engine was stripped, there were no exhausters, various other bits were missing but he knew that to many enthusiasts, re-vamping the "prototype" Class 14 would be sacriledge. Nevertheless, scrapping it would have been an even bigger shame and without a doubt that was the alternative fate for the loco.
Similarly, the class 03, bought with the remains of a Deutz air cooled engine and hydrostatic link installed, was not be refitted with Gardner and fluid coupling but has been retro-fitted with a Cummins and 2-speed Twin Disc hydraulic transmission to be a more practical return to running order.
"Main line" locomotives were generally treated as a "class" and modifications were therefore carried out on all examples and fully documented - but on the industrial market many locos were built as "one-offs" from basic designs and individual owners might choose to modify their loco to suit their requirements. For example few industrials were built with any form of train braking, but as roller-bearinged stock became the norm many were retrofitted with train braking for safety and practicality. Take "Cheedale" as an example. The standard Thomas Hill design had a planned space at the front of the casings so that the customer could have either a train air compressor or vacuum brake exhauster combined with the Rolls-Royce C6 engine, OR the C8 (8 cylinder) engine. Examples with the 8 cylinder engine were built for Shell, and the 6 cylinder for the MoD (who later came back and asked for an additional train air compressor which was retro-fitted in front of the radiator as it was not originally called-for. ICI came and asked for everything the 8 cylinder engine, exhauster and compressor, resulting in a loco with an extended casing but still immediately recognisable as a Thomas Hill chain drive Vanguard.
Those alterations were performed by the original manufacturer, but Andrew's 03 is also a case in point. Not only was it re-powered on site in Belgium but - presumably because the owner was unaware of what weight the loco ought to be or the implications of increasing weight on to axles and springs - roughly 10tons of steel slab was added to the loco, scaling it at just under 38tons
Moreover, the vast majority of industrial locos were not - in this country - expected to do "tripping" duties. Most industrial sites were closely connected to the national network and the typical industrial probably averaged only walking pace through its working day. Indeed, from a safety point of view some industrial operators would not want their locomotives able to travel at speeds which, within the factory/quarry/mill/sidings would be dangerous.
But when put in a heritage railway environment operators seem surprised to find that the loco won't do 25mph without overheating the transmission, and often such locomotives are casually dismissed as a result. Of course, they were not built with that sort of work in mind, and if they are to earn their keep it is not unreasonable that alterations be made to suit, and in many respects this is in keeping with the philosophies of their original manufacturers and owners.