I do not know what the Thermoengine looks like but I have seen people take a 80mm fan grill turned upside down, so that the ends that you would put the screws through face up, and mount it to the HS with some long bolts and washers. You run the bolts through the edge of the grill where they can grab onto the fins of the HS. You may have to try a few different size bolt to see which ones grab the best. (Dont forget the washers). you then take your 80mm fan and mount it to the grill with the holes that you would normally use for mounting the grill. Good luck.
the thermo engine is a curious design, the dead spot where the motor is that hampers performance on most fans shouldn't have that much effect on the thermo, I can't think of a way in which you would attach it using a grill like some people do on most heatsinks, I would surgest that you make you own shroud for it out of a biscuit tin, first start of using card and make something similar to the standard shroud but obviously to fit an 80mm fan, shape it kind of like an upside down piramid, then when you have finalised the design use it as a templet on a flat peice of thin metal (biscuit tins are great for this as they are easy to cut with metal cutting scissors) and cut it out and fold it into shape, I made my own for my FOP 80, as soon as I get my capture card I'm going to write an article on it.