If you're thinking the LS-X swap is a bit mainstream...
On eBay: Jaguar V12 Mazda Miata at US$5,900
This Mazda Miata cost over $20,000 to build in 1995, plus basic car cost. The V12 Jaguar engine not only has 6 downdraft Weber exact copy clone carbs (Made by EMPI), it has been ported and polished, 12.5:1 compression, custom pistons, balanced flywheel, modified crankshaft, aluminum racing radiator, AND THE MOST INSANE CUSTOM EXHAUST HEADERS - seriously expensive !!! The frame has been professionally modified and strengthened. Front cross member supports the V12 nicely.
I drove this car about 90 miles to this storage location 3 years ago and parked it. It ran fine, but it was misfiring slightly at that time. Now it does not start. Generally when a Jag V12 has a misfire it indicates a blown head gasket. I believe this is the problem, but there may be other issues. The engine / car was built in Florida. I drove it for a couple of radical months in the Vegas area. It is totally insane !! Forget first gear. Spins wheels easily in 3rd gear - massive torque and horsepower. Makes all other V12 Jags look wimpy, and I own two 6.0 XJS. I had planned to install a Ford rear axle with 2:87 gears, or similar.
There is a stereo face plate included with the sale. The top is shot and the interior needs cosmetics. Tires poor. Windshield has crack from my hitting it accidentally. No windshield wipers - no room. Clutch and gear box are like new. The Tokico coil over load leveler shocks are high value. Car sits perfectly flat and steering is tight. Exhaust is 3 inch and in excellent condition. 5 speed T-5 trans is perfect.
Replacement value on the 6 EMPI double downdraft carbs, with K&N air cleaners, velocity stacks, linkage and manifolds - if you could ever find a set of manifolds for a Jag V12 - would exceed $4,000.
I have held onto this rocket for a couple of years, hoping to get the time to build it out. Not happening - no time. The race engine, without the carbs, is worth almost double the asking price. I may be willing to separate and just sell the carbs,or the race engine and T-5. Best deal is to buy the package. And finish putting together The Worlds' Wildest Miata.