The VW/Audi compressors are bit on the large side, although much better quality than the 'el cheapo' tyre compressors from other sources. For a start they're well balanced.
They're made by a company called Valeo, and the German modellers like to use a pair of them for their big 'pressluft' boats, although one would be more than sufficient for a smaller boat as they shift a lot of air.
If you were to replace the standard motor with something a bit smaller, and lay it on it's side, then you could probably squeeze it in.
I've no experience of the Robart cylinders, but they look like standard aerosol canisters (probably aluminium). For a compressed air cylinder, which will be sucking in humid air, I would like a drain plug to enable moisture in the tank to be drained out.
I would think these cylinders are likely to be a tad too thin to enable the fitting of a plug, but even assuming there was enough meat on them, any alteration of the cylinder like this will lower the pressure strength of the cylinder. Definitely an area to over-engineer.