The spar was designed for a calculated stiffness, based on total length and support. One of the reasons that NA's like the keel-stepped spar is that weight is minimized and the section can be smaller for a given length.
At some point of stress, that shortened mast will fail, and fail sooner than the NA would have calculated for a stock boat.
Remember that the I-36 is an off shore cruiser/racer and built and rigged for that purpose. That's just one reason it is rightly referred to as a modern classic.
Hypothetically you can safely convert any keel stepped rig to a deck stepped, but you have to change the cabin/bulkhead engineering & structure midships and change the spar section... This is not cheap or easy.
Any knowledgeable surveyor will red-flag such a modification, unless there is actual certification that all the internal work was done and a new spar section was installed. (Possible, but rather unlikely.)
And then, way down the road, someday... if the rig/boat is damaged by sea or wind conditions, your insurance company will probably use that change as a huge loophole to disallow your claim and cancel your coverage.
And, As for splicing it back together, note that a section is missing. You would need the exact missing section, probably over a foot long, and then a plug to exactly match the inner shape. The section is probably not possible to find, after all these decades.
Better think twice before buying that boat.