I used to own 4G92P and swapped with 4G91 head. Also I've done the 4G93P (SOHC Wira) with 4G91 DOHC head.
Others than the timing belt, you also need a new mounting block support. This is necessary. Head gasket unless u r using a metal gasket. Also you change your bearings too.
U definitely need a 4G91 distributor, you can use the carbie type. Cost about RM150 less.
Last time I did some polish (not porting) cost me RM150. This is extra job, not necessary,
The 4G91 head has a slightly smaller port of the water channel therefore you will notice that after swapping the head, your engine will run slightly hotter but not into a high risk level.
After done the convertion I Found that my car revs higher and smoother than SOHC. Fuel consumption slightly improve, the reason not the DOHC, the reason is my leg cos it's easier to rev up the engine so I rev less. After the conversion I've drove it more than 9 months without problem. Overall I've spend RM1400 for the conversion and I nvr need to drive back to garage. Not even a single problem. Therefore I'm happy.
My bro. last time kena tipu by one of the famous mechanics in Shah Alam and he sold my bro. a 4G91 head instead of 93. Last time I don't know how to see the different, but now I have learned the differences. After the conversion that mechanics also swapped my bro. intake manifold, he has changed to 91 air intake manifold. The differences is 91 has no EGR while our local 92 and 93 has EGR. Damn, once open the head, I've seen a lot of carbon debris sticking on the top of the piston.
Finally, my mechanics suggest me since this is 91 head, what he can trying to do it do a port and polishing. Use the venturi gasket and mark the a porting size on the 91 head and port it back to 93. Although it takes a long time and after porting I went to check it and I see no different of the sizes except the head has been port and polish. Also the only thing cannot do is the middle of the inlet valve porting size and the cams profile as well as the spring. Originally the 93 head, the valve spring using 5mm longer spring than 91.
After rebuilting the head my brother car is running fine and without a single time going back to the garage but recently I heard my bro. said he notice a quite a loud noise of from the tappet of the head. I think it's time to clean the tappet. My bro car is an auto. Last time his car 4G93P SOHC can run max. 165km/h and now can reach over 170km/h, without touching any tuning of the car.
You can change the head to DOHC but there's one thing you hard to change. It is the ignition coil. The 4G93 DOHC head has different configuration of the ignition coil. Obviously when you open up the bonnet, you could see the distributor at the left hand side of the head.
The 4G92 and 4G93 block has the same diameter of the cylinder. The 93 block are 20mm longer than 92 block. Therefore I believe the piston circle on the head is the same. 91 has smaller piston circle.
Well, that's my experience and all I can share.
any1 interested can PM me ur best price k n we can discuss....chers 
