I’ve put a petrol 2.3 from a 90 in my Series III I suspect it will be the same.
It depends how you define “Drop in”
The bolts will be metric, where as unless you have a late Series III yours will be imperial.
In theory you will have to enlarge the bellhousing holes to fit the metric studs. Mine fitted without, but of course I had to buy new nuts.
It had an electric fuel pump so I had to fit the Series mechanical one. The Series pump had a thicker side plate, so the metric bolts were too short, so I had to buy new bolts (this is getting expensive, but could have been a problem on a Sunday afternoon!).
The left had engine mounting is in the wrong place, but on the 2.3 there is a second set of holes that mounts the mounting ion the right place for the Series III. The plastic fan looked very close to the radiator, so I used the metal one. I think the bolts were too long, so I had to buy another 4 bolts! The bottom hose is different, I managed to heat it in a saucepan and stretch it over a broom handle till it fitted (I hope it never fails at the roadside, looks like an AA job!) I think I used the Series thermostat cover (More new studs?)
Manifolds. I took the easy way out and used the Series manifolds and carb that then bolted straight on to the exhaust. Obviously I will be loosing a bit of power, but not enough to worry about. I know people running 2.5s on the Series carb etc, probably loosing out a touch, but still better than a (knackered) 2 ΒΌ! Had to change the breather pipes a bit.
The Ninety exhaust is a different diameter, so the exhaust won't bolt onto the 90 manifold, and the 90 front pipe won’t fit the Series exhaust. There are lots of ways round this depending on your skills (and/or how much you want to spend)
I have recently been told that if you cut the Series front pipe you can fit the bigger 90 clamping ring and it will then bolt (& seal) onto the 90 manifold. Obviously you will need to weld up the front pipe.
It’s a few years since I did it, but I think that’s about it. Not quite “drop in” but nothing difficult, and no welding (depending on what you do with the exhaust.) The hardest part was fitting the bottom hose!