I think it would be a interesting experience to try a Gilbert 11 as a diesel. But it may work fine without a special diesel head and adjustable compression. When i get around to it I'll give it a try and see what happens with one of my Gilbert engines. I was thinking that one might be able to just drill and tap for a screw to go in and maybe you could use just a screw without a contra piston to change the compression just a little bit, but I also don't know how well that might work either.
But you are correct in that the engine doesn't look very strong for this purpose. So it is probably a bad idea.
The Gilbert glow heads can be converted to a screw in glow plug easily. You actually get a slight improvement in the compression ratio as the Gilbert heads have a large recess for the glow plug element. A screw in glow plug reduces this recess void a little bit.