all the models on grabcad of stirling engines that i checked were done in solidworks, and in order to convert them to a format i can use, you need to have solidworks. so, i'm building the engine myself using them as a guide.
and in a while, when there aren't other things that take priority, it very much seems that adding this model of a stirling engine, in .stl format, to grabcad, would be very useful to other people who don't have solidworks.