Keep active engines in the purchase list.
Configuration file: enginespersist
Command line: -P
There are some problems with engines becoming obsolete. For example, there are no more helicopters available after about 2030 so heliports become obsolete too, or you might want to have a particular engine even though there are newer ones, maybe because it is relatively cheap but sufficient for a certain purpose.
Or perhaps you want to be able to buy steam engines in 2000, which would cruise along scenic mountain ranges... (thanks to Wolfgang Preiss for that idea!)
What this patch does is not to let any engines in use become obsolete. Any engine that exists in the game can always be purchased. With this and Cht: Year (or EternalGame), you can basically continue a savegame playing forever.
Also, the reliability remains at the peak, with the reasonable assumption that if you still have it, you'll also have some engineers who know how to repair it, right? An engine doesn't just get less reliable because it has been designed a hundred years ago. It might not be up to par in all the specifications, but why should it suddenly start to deteriorate even if you just bought it?
Only after the last of its kind have been sold will it start to deteriorate, and eventually disappear, because nobody knows how to build and properly repair them anymore. If you really want to hang on to some engine, make sure you always have one in stock, maybe in a depot somewhere. It just needs to exist, it doesn't have to run at all.
This patch will also re-enable the purchases of engines that you still have but can't buy anymore.