If you are only getting warning messages, not error messages, then they can be ignored.
In general I would never advise anyone to "solve" a problem by increasing the build number of an asset. The build number tells Trainz which compliance tests can be applied to an asset when it is loaded into Surveyor/Driver. So by increasing the build number you run the risk of applying different (and perhaps even stricter) compliance tests that the asset may completely fail, producing an error instead of a warning.
A few questions to help me understand your problem.
Do the engines and rolling stock items that you cannot place appear in the Trains (F7) object selection tool window?
Are they showing as faulty or with missing dependencies in CM? (warnings do not show as "faulty")