Instead of a reverse rotation gearset, you could use an 850 cam which are interchangeable. You might want to spend the money on a hotter 850 cam instead of the gears. You *might* be able to sell the A112 cam to someone who wants to reverse the rotation of an 850 motor for use in a 600. An Abarth Autobianchi A112 1050cc engine uses a cogged belt drive for the cam so reverse rotation gears won't work. Not sure if you can modify an A112 engine with cogged belt to chain drive. I think you can because you can buy a cogged belt drive kit to install on an engine with chain driven cam. One version of the A112 engines does have a chain drive cam.
Using an 850 cam, you'd need an 850 distributor due to the direction of the distributor drive gear on the cam or you could remove the gear off the shaft of an 850 distributor and install it on the A112 one (cheaper too). Be very careful if you do this so as not to damage the gear. I have several used 850 distributors if interested. If swapping distributors/parts, note that the 903cc and A112 engines use a longer shaft.
I also have an 850 292 degree reground performance cam for sale. It's newly reground and got it from Germany a number of years ago. An 850 cam would have the lobe for fuel pump and it might need to be ground down for clearance. I just sold a set of the reverse gears a few weeks ago to a forum member. The "new" Scuderia Topolino in Germany has the gears too, listed at 229 Euro. Berni Motori items aren't inexpensive and I think you still need to go through a US distributor here.
I have an 850 903cc block with A112 crank, cam, rods, pistons & head. It's basically an A112 1050cc engine in an 850 block and looks stock 850 from the outside. The rotation was reversed (from 850 rotation) to mate to a 600 transaxle. O gawd, at my age now, I can't remember all the things that were done to the motor...
I had it built by Paul V. before he moved back to Europe quite a while ago. I have a vague recollection that the snout of the A112 crank needs a minor modification. I think what it is, is that the 850 transaxle input shaft works with a bronze bushing in the snout of an 850 crank. It might be that the A112 has a roller bearing with different diameter?? It should be immediately obvious with both cranks side by side.
Not sure if you can use the A112 flywheel as-is or you need to use an 850 one or if the clutch covers are interchangeable (could have different mounting hole locations?).
I could be wrong but I don't think you need to worry about the direction of the pistons/rods in an A112 engine. I'm pretty sure A112 pistons don't have a directional arrow on them. If you do pull the pistons out, make sure to mark the order so they go back in the same cylinders. Of course, inspect the bores for wear. It might be a good idea to remove the head regardless for inspection and remove carbon deposits if needed. Unless it's a low mileage engine, I'd inspect the rocker arm shaft and rockers for wear. The underside of the shaft can be badly scored/worn sometimes, esp. if oil wasn't changed regularly. If you need to have the head surface machined, make absolutely certain to remove only the minimal amount of material possible (like a few thou. only). Many heads have been ruined by taking off too much material by machine shops that don't know what they are doing.
You'll want a clutch cover that can rotate in opposite direction. The fan blade on the alternator is rotational. You may find the A112 one works fine. From what I can see so far is that you can't just swap an 850 fan blade on as the hole for the shaft is a different size. I have an A112 fan blade that won't fit on an 850 alternator. A bushing might solve this or drilling out the hole in center of the blade.
IIRC, an A112 engine has higher compression and you'll need the appropriate grade gas. A Euro distributor may work better? You might want to check the plugs to see if you have the right heat range, esp. if you use a different profile cam (ie., 850).
The Abarth A112 engines I believe use an electronic Marelli distributor as standard. If your engine doesn't have one you can buy these. A mechanical dist. would look more original though.
An A112 engine has the oil filler cap on the rocker cover at the opposite. Use an 850 cover. Not sure what carb an A112 uses. You would have a number of choices and there are aftermarket intake manifolds so you can use a different Weber. I think it is the Abarth A112 engine that has a one-piece rocker cover + intake manifold so if you use an 850 rocker cover, you'll need a new intake manifold (stock 850 or aftermarket).
I think this pretty much covers it - I think... The A112 engine will have 70 HP stock which will be a lot more than the original engine. If that isn't enough tho., you could always do a few performance mods.