>> Lots of airplanes use a 180 HP O360. What would involved in converting them to IO360 for +20 HP?
Yeah, doesn't work like that! The 200 HP IO360's have angle valve cylinders... the angle give more room for larger intake valves, hence higher volumetric efficiency... the angle valve cylinder gets something like 93% fresh air/fuel charge in on each intake cycle, versus something like 83% for the more congested intake pathway O360 parallel valve engine. That's responsible for MOST of the power output difference between the engines.
The other piece is the difference in compression ratios... 8.5:1 in the parallel valve O360; 8.7:1 in the angle valve IO360. You can buy higher compression pistons for the parallel valve engines... and higher compression ratio pistons for the angle valve as well... Firewall Forward has an STC for 10:1 pistons in the IO360 that delivers 225 HP in the Cardinal RG and Mooney... but, for certification reasons, it's flat rated to 200 HP with a manifold pressure redline around 27". I've heard that some pilots "forget" to observe that redline. But, even with the redline, that means you maintain 200 HP as you climb by opening the throttle to maintain redline MP.
>> Obviously, intake system.
>> Same block and internals?
Well, these engines don't have blocks... they have a crankcase, and individually attached cylinders. You'd have to do some parts catalog perusal to figure out if there's a Lycoming-endorsed configuration attaching angle valve cylinders to your crankcase and crank... probably easier, as someone noted, to just purchase the engine you want.
>> Would the STC be airframe specific for Certified?
There's two STCs. One to modify an engine, if that's what you end up doing. And then a second to install the modified engine in your airframe. You can skip the first by just choosing an existing Lycoming engine configuration.
>> What about Experimental?
Well, experimental aircraft can roll-their-own when it comes to configuring engines. If you mean making your existing certified engine experimental, the desirable, flexible experimental category is experimental homebuilt... and you don't qualify for that. There's also experimental flight test and experimental market research, and so on. But those are all VERY restricted in what you can do with the plane, and where you can fly... and typically only endure for a limited time. Not practical or useful.
Paul