I too agree with almost everyone here, to some extent.
One difference perhaps being, i HAVE made a riding mower do what it was not supposed to do.
It was sort of a big project even though it didn't cost much. I put a small push mower engine on pushed as far back as possible to make room on the front of the frame, reinforced the frame (after bending it) with welded angle iron (but never bothered to run the self-tappers back out lol). Moved the front axle all the way to the front, lengthened the steering rod, added thrust bearings under front spindles, matched up different wheel halves on skinny 4.10-4 tires to tuck the front tires in as close as possible to reduce scrub radius (to minimize steering effort), moved front axle up to hit frame to defeat tilt/pivot (to mitigate tendency to tip over if back wheels come up) built a throttle bracket to idle the engine down, used an ac accumulator as a muffler, used a fan clutch as a flywheel on the engine, pressed a drive belt pulley onto the pushmower blade hub, added 4 bolt wheel adapters to run car tires filled with liquid, another liquid filled wheel on the back, heavy brake drum behind the 'spare' tire for weight, brake rotors behind the drive wheels for weight, stock ~3.5gal fuel tank filled for weight, notched the dash to stow the crane under the hood for weather resistance, etc etc.
In the end it DID work well and could do things my FEL tractors I bought later could not do, like deliver engines through 30-something inch wide openings. Moving engines across dirt and gravel is what i built it to do, it definitely does that. I have had 500lbs on the crane and the back stays down without me sitting on it.
So sure, if you want to then go for it. But understand you might get more out of the learning process than you do out of the machine you'll build. But as far as building things, you gotta start somewhere, and it helps if its cheap and the stakes are low! That's what made me pull the trigger on this thing. I had everything except the $130 crane, and the 4 bolt hub adapters. I put maybe $250 into the conversion, and the rest of the major components i had sitting around.
I also own a garden tractor and a SCUT w/fel and if you want to skip the fab and just get work done you should definitely go that route!
The Case i got with no engine for $250, put a $99 HF predator 212 in it, and made the missing 3pt parts it didn't come with out of scrap car parts. It's vastly more capable (3pt can also pick up about 500lbs, just not as far out because there's no weight on the front) than the crane riding mower. I have lots more mods in store for it.