Yea, strip everything off your 1.9l and put it on the 2.4 block and bolt it in. Thats all there is to it. I'd suggest a 2.0l/2.2l or ported 1.9l head tho'.
I have a 2.2l with dual sidedrafts about ready to be installed in about a week or so. You can come down and check it out if you like. What part of Seattle are you in? I'm in Federal Way.
bodie38 is right....I didn't mean to over simplify it. But it really is no more difficult then pulling a 1.9 out and putting it back in. Much less headache then a non-Opel engine conversion. Bodie38 is right about the internals between the 2.2 and 2.4. I chucked the 2.2 pistons and put in Venolia pistons (bored .040 over) instead. The 2.2 pistons tend to break at the skirt.
I have a 2.4 in my GT and I can answer a few questions. I didn't do it myself so some of this is not from experience but came up in my research.
The 2.4 has higher quality internals (forged, not cast) than a 2.2 so I think it's worth the extra bucks. somebody else can speak to that better than I.
There is no intake manifold for a single carb that will fit the 2.2 or 2.4 head (the exhaust manifold/header is different also). That is why people use 1.9 and 2.0 heads. You can get dual side draft manifolds for 2.2 and 2.4 though.
Mine has a 2.2 head and Jetronic-LE (I think) injection. Jetronic-L fits 1.9 and 2.0 heads. I was told that Motronic Injection is too tall to fit under the hood without changing from stock hood.
Pretty much everything from the 1.9 will bolt to the bigger blocks but you will probably need a better radiator and a stronger transmission. You should get a bigger clutch and flywheel also(and brakes!).
The 2.2 and 2.4 clutchs are kinda pricey so you may want to research the S-10 clutch mod.
I was told the four speed wouldn't stand up to the extra power so I got a Getrag 5-sp also. That required a modified driveshaft and linkage.
I love the car the way it is but in hindsight if I were doing it again, I would have used a nice 2.0 head with a twin 38 Weber carb. It would have been less effort/cost. The injection is smooth though! Good Luck!
Why would I go with a carb? Simplicity. Yes the FI is more fuel efficient and more power- as long as it's working. I'm no mechanic, but I can replace a carb, and it would be easier to find someone to fix a carb than to fix an injection problem on a system that wasn't ever sold in the US. Yes its Bosch and very similar to other Bosch systems but most mechanics hear Opel and say "What's that". If you're an FI expert go for it and give me your email so I can ask you questions when I try to start it up after sitting for 8 months