Here is the topic then: What's a good way to keep a cockpit warm?
A) As others have posted, get the heat moved in as effectively as possible, and do not let it leak out or get away. Insulate and stop air leaks. For a LSA, insulation carries a weight penalty, but door seals and tight construction does not have as much of a weight penalty.
What about the new LSA's, do any of them keep you really warm?
A) As others have posted, a well built LSA with good source of heat can stay warm.
Does more horsepower give you a better shot at staying warm?
A) Yes, a larger engine that burns more fuel per hour will have more waste heat that might be used to provide cabin heat. Also, waste heat from water is easier to use and less risky than hot air muffs on mufflers. However, water heat will have a slightly higher weight penalty.
Every design decision has some sort of compromise or trade-off involved. Great question, and I hope others post so I can learn from others.