Concept: we know XXL fusion engines weight and XL engines are half of the weight. so I thought why not make some more exotic engine types? enter XL Fission Engines, XL ICEs, XL fuel Cells and XXXL Fusion Engines. These engines are lighter but bulkier than normal engines of equivalent types.
XL Fission: 50% weight but takes up 3 crits in the side torso's. Only give five free heat sinks instead of ten.
XL ICE: 2 frees heat sinks, 50% the weight but same bulk as IS XL fusion engines. ICE Limitations apply.
XL Fuel Cell: 2 free heat sinks, 50% of the weight of Fuel cell engines but just at much bulk as IS XLFE's. Fuel cell fuel tank rules apply.
XXXL Fusion Engines: 10 free heat sinks, generates 2 heat when standing still, 8 heat when moving, Takes up majority of side torso slots in a mech or 4 vehicle slots. 12.5% the weight of the SFE. Tonange Rounded down to nearest half ton.
This is Just a rought draft, but I think they can be worked on if we put our heads together.