A major difference you should keep in mind, is between the K3VE, and K3VE2
The K3VE is a 1.3 with a 10:1 compression, 87hp and 120Nm. It was available in, for example, the YRV and Copen. It’s also known as the 2SZ for Toyota models, like the Yaris. Although I’m not sure if they are completely interchangeable because of mounting and sensors etc.
The K3VE2 (to which you were referring) is a high compression variant (11:1) with 102hp, 120Nm, a higher redline and was fitted to the Sirion M1. From what I’ve seen, the higher compression is obtained with different pistons, maybe rods as well, I’m not sure.
The Sirion 2 (M3) was first fitted with the same K3VE as mentioned above, but later updated for the facelift model. It gained no more torque, but 4 hp (91hp) thanks to fitting the same cylinder head as fitted to the 3SZ-VE
The 3SZ-VE, also fitted to the Sirion M3, Materia M4 and Terios J2, is the 1.5 variant of the K-series/SZ-series. It is, like you said, basically a stroked out version of the K3VE (NOT the VE2). It also has a 10:1 compression and puts out 103hp and 132Nm.
When I took apart a K3VE (from a YRV) and 3SZ-VE next to each other, one thing I noticed is that the timing chain and sprockets are different. I believe they were wider and featured a different amount of teeth. This might be interchangeable, but I’m not sure.
Timing chain cover K3 vs 3SZ
Another (quite annoying) difference is the height of the 3SZ block. I think it’s about an inch taller, which makes clearance a bit of an issue. It fits, but barely. I used the intake from a K3VE2, because it has that instant 90° bend on the throttle body, and even that is slightly hitting the bonnet.
Then there’s the ECU. The way both engines are controlled is quite different. I know someone on this forum (was it @thefiztec ?) used a K3VE2 ECU to run a 3SZ, which worked apart from a rough idle. I’m not a huge fan of using different ECU’s, so I built my entire wiring harness so that I could use the stock 3SZ ECU. However, both cars used a 3SZ with it’s stock head, so there’s no way of telling what it would do if you’d change the head.
A summary of headaches when you’d put a K3VE2 head on a 3SZ:
- Clearance issues (engine fitment inside the car)
- Timing chain/sprockets
- Clearance issues (valve vs pistons)
- Compression (because the 3SZ block still has a 10:1 compression, and quite different internals)
- Engine management (especially ignition)
- Revs (will the 3SZ internals be able to rev up to VE2 speeds?)
Conclusion: I think it’s best to just source another K3VE2 block and swap it in. Doing a head swap probably requires more research, testing, money, and almost certainly a standalone ECU. I’m not saying it can’t be done, but I think for you it wouldn’t be worth the trouble. I hope this helps!
If you do want to look into this whole story a bit more, I suggest you read this:
Good luck!