First up - I'm not a lawyer and my comments are my personal understanding and opinions only, gleaned from a reasonable amount of online research.
Now, most copyrights are covered under 'fair use' and would likely allow for personal copies (such as backups or working documents) to be made if you have a legal right to the original (as in you currently own the original magazine and you want workshop copies of the plans). If you were to give or sell the magazine to anyone, for any price, you no longer own the right to use that material and the copies you made are no longer legal for you to possess and you must dispose of them. Similarly, if your neighbour makes copies from the magazines he purchased from you (which is legal) and then sells the magazines back to you, he now needs to dispose of his copies as he now has no legal right to own them.
If you make a copy of the copyrighted plans and give them to anyone, regardless of that persons' usage of the plans, yes, you are breaking the law.
A lot of plan sets, as mentioned previously, only allow for one engine to be produced from each plan set. This means that if you buy a set of plans with this condition, then build an engine from them, you are breaking the law if you then on-sell the plans to another person. I have no idea what the exact terms the magazine was sold under so not sure if this applies to this publication, but it is a common clause on many plan sets.
What most people seem to have issues with is the 'but it's out of print' bit. Categorically YES, copyright still exists after the item in question goes out of print and extends for a very long time (it varies somewhat depending on the media type and the date it was produced). People seem to be concerned that access to such items is going to be lost (and it may well be lost) and consider they're not hurting anyone by breaking the copyright on such an item. Now consider the recent case of Ron Colonna's Offy book/plans. This book is out of print and (as far as I know) will likely never be reprinted, so some builders decided to disregard copyright law and try and copy/share the plans, obviously thinking they weren't hurting anyone. As it turns out, Ron was in the process of producing an electronic version of this material and would have suffered substantial loss if his plans has been disseminated to the public domain.
We simply cannot know what the current copyright holders of these "obsolete" plans have in mind, now or in the future. They may be lost forever, or stored for future use, or possibly being compiled into a new publication. We don't own these copyrights and we cannot make decisions on what is to happen with them just to satisfy our own selfish desires. Surely no-one here would attempt to breach the copyright and share one of the plans designers such as Brian Rupnow or George Britnell sell so why are the rights of these unknown copyright holders not held to the same standard?
My suggestion - if there's so much interested in the plans for this engine, then there's an opportunity for someone to create a high quality, original plan set for this engine and sell it themselves. Now if that sounds like a lot of work or just too difficult (which I wouldn't disagree with) then consider how ethical it is to steal the existing product from the person who did do the work.