Any pictures? The amount of work depends on the type of sealant they used. If it is a latex or silicone base caulk (most likely), then you can easily remove it with a few razor blades. If they used an epoxy type (liquid nails, etc.) it may be a bit harder to clean the residue off. If the blades cannot cut through it, then maybe try a scrapper or a 5 in 1 and chisel it off with a mallet.
Yes you can find many different vents and skylights on several vendors sites. Including by not limited to vintage trailer supply and Inland RV. Plan on buying sealant along with the vents for adequate water proofing, and always rivet them secure.
Custom restorations and renovations
Keeping the Art on the road, one project at a time