Is there any suitable articulated diesel unit on the market anywhere that would fit within the UK loading gauge and not fall foul of the emissions regs? If there is hopefully some could be ordered then all the pacers could be turn into baked bean cans
Finding some sort of diesel engine shouldn't be a problem, despite claims that it's impossible to build a compliant engine, etc. Cummins have a compliant QSK19-R, for example, Vivarail are trying out Ford Transit engines, Perkins have a 6 cylinder 400hp-ish product which might be convertible to horizontal use (if someone was willing to pay, and they could adapt their horizontal tech from the 6.354/2006 ranges), Hitachi have a MTU module, etc.
The major issue is convincing someone that there will be a return on their investment, to get them to commit to the huge amount of cash needed to order a bunch of new DMMU/DHMU/DEMU units. ROSCOs won't do a thing unless a TOC or DfT is prepared to basically guarantee that return.
Personally, I think it's looking more likely that cascaded non-Pacer DxMU units, including HSTs, will be how the Pacer fleets are gradually withdrawn. There's a lot of electrification going around, and that should free up a lot of diesel units. DEMU-ified D78s might well feature in that, if the product turns out well.
I wouldn't rule out a small DMU purchase somewhere, but it may well be a high end high speed express product, displacing older and slower DMUs downwards, rather than a Pacer replacement. That's probably a better long term prospect, I reckon. A cousin to the 700, 800, or new ScotRail EMUs could be the answer if there's enough demand once you shuffle everything else downwards.