But still, only one part has to be changed in the engine. Which is what Ward Duncan said.
In contrast, when they rebuilt that XP-82 they were very lucky to find a suitable left turning Merlin engine. I rankled some people I guess when I pointed out that for the Merlin you needed a Completely Different Engine for the left turning installation. In aircraft maintenance terms a V-1650 that turns the other way is a different part number and nothing can be done in the field to change that. The fact that it has a lot of parts in common with the other more common engine does not matter.
Actually not true. I worked in an Allison shop for 3 years. In that time, we built up 13 Allisons. A couple were left-hand turn engines, one for a P-38 and one for a Russian aircraft. I say "we," but the main guy was Joe Yancey; his shop and his expertise. Joe DID build a LH engine for a Russian Il-2. It is in Paul Allen's museum in Everett, WA.
There is an idler gear that has to be added. It makes the camshafts turn in the same direction as a right hand engine when the crankshaft is turning the other way. So you did NOT have to change magnetos because the camshafts turn the same direction for LH and RH engines. In the LH prop rotation drawing above, it is labeled "K." That is one part that is different between LH and RH engines.
You have to flip the crankshaft, but nothing on or in the crankshaft changes ... it is the same for LH and RH engines.
You need a starter that turns the other way. That is the second part that must be different. The starter also requires a starter slip gear that goes the other way. That's the third part that is different.
You need a completely new wiring harness because the crankshaft was flipped, so the middle two throws go the other way and the two inside cylinders (cylinders 3 and 4) swap ignition wires. That is part number 4 that is different. If you don't change ignition harnesses, the engine will spit and sputter when you start it, and you will NOT be able to make it run smoothly because the center two crank throws will not be timed correctly.
In point of fact, it is trivially easy to make either a right or left hand turn engine if you are starting from parts and building an Allison up. But, if you have an operating right hand engine, you have to disassemble it and flip the crankshaft to start to CHANGE from right to left turn, and add the parts above. Not especially difficult, but it is time-consuming. Takes probably 2 - 3 days with 2 people doing the work in a clean room. You COULD do it all mostly in one day in an emergency, but it is far better to have spare left-hand engines than to build them up as you need them. The P-38 is the only airplane that normally uses a left-hand Allison, so if you have spare right-turn engines for your P-38(s), you SHOULD have the same number of spare left-hand engines.
I'm not guessing this, I have participated in building them up from parts, borke them in, and saw them fly.