Well I'm being contrary here but I would sell the Maverick (if you could truly get that much out of it... selling modified older cars is tricky) and keep leasing the Smart car (though I have no love for the Smart car itself). Yes you are "renting" the Smart car at $163/mo but you also have zero maintenance costs, and no depreciation to worry about, and no cost for turning the car back in early (which you'd have to count as a cost towards a different car). I'd keep the Smart car, put some money in a car "replacement" fund for 2 years, and then look at your options at the end.
As an aside, I love car stuff, engine swapping, etc., but it is generally not ever going to make sense cost wise. Every swap I've done has taken much more time and money than anticipated and the benefits generally less than planned. I'm planning an NV4500 (transmission) swap in my 1974 International right now, to get away from a "trucky" shifting transmission, and to get an overdrive gear. It will probably be around $3k when all is said and done and at that price it would probably take a hundred years to get that cost back!
