The spar is actually two halves but it is bolted together with a spar cap bridge which is about 1/3rd of the wing span. it is a huge deal to remove the wing and replace a hundred rivets to replace this piece. Usually due to corrosion. To trailer a Mooney, you drill the rivets all along the tailcone at the rear bulkhead and separate the tailcone from the fuselage. remove the tail from the tailcone, and place the Mooney on the trailer with the wings down the trailer.
Easier to gain speed from drag mods than horsepower. The cowling, windshield, and things like antennas, door handles, and fairings are worth 20 MPH. It would take another 50-80 horsepower to get that.
I have flown the C, E, and J. I'd guess the MP is about the same with ram air on all 3. The ram air off chokes down the vintage mooney's a couple inches due to the poor airbox design. the J I flew long ago woulnd only gain 1/2" or less. on our 77 J it is deactivated.
You can mod an E, F, and even a C model with a full 201 kit if you wish. Everything goes. Maxwell doesn't recommend inner gear doors on Johnson bar airplanes due to the effort required to retract the gear with that air load. I know someone who has the 201 windshield and cowl on his 66 C model, it even has the fuel injected engine It is also exactly as fast as my J ~155 KTAS. Except the longer rear window it looks the same as well.
The short bodied C and E models seemed definately to get off the ground quicker and the E model when solo climbed like an empty 747 compared to a J. Cruise flight seemed about the same, perhaps the were a little more sporty.
For hot dogging I liked the E a lot better. For everything else cheaply, the J is it. The only thing better than a Mooney is a turbo Mooney!