Sometimes layered files/templates, have features included such as grouped layers and such, that older, simpler programs like GIMP or PSP7 do not support. Hence they won't open.
To remedy that, I usually open them in a later version of PSP, then save as a PSP7 compatible file ... you loose some features, but it doesn't really affect the template (unless there are more than 100 layers - then any layer above the 100th gets lost with a conversion to PSP7 compatible).
So, unless each roundel on each wing, upper and lower, and each aileron on each wing, upper and lower has it's own layer and so on, there is little to worry about. But should that be so nonetheless, then in the more recent version of PSP, you either need to delete some unessential layers for your needs, or merge layers together, so also reducing the number in total.