storing spare engines

Make sure all holes that are external are blocked up...either with plastic plugs or very oily/greasey rags.
Caterpillar have this stuff called VCI Oil....it's made for just the purpose of preserving engines, diffs and trannys.

[ame]https://catoilandgas.cat.com/cda/files/1823203/7/PEHJ0241.pdf[/ame]

We use it ^^^^^^^^^^^^ all the time at work.
You may be able to find something similar.
Pour a pint of it into the sump, and some in each spark plug hole, and a little down the carby.