if this topic still open, i wanna share my findings ..
i don't really understand or have full knowledge about engine ..
but several of my friends (they are involved in street racing, modification, other engine stuff) said, to convert SOHC to DOHC requires the following ..
Twincam Head- 4G91 or 4G93 (93 is better because it is slightly taller, means can tarik panjang)
Wiring - Dohc wiring with Dohc ECU (if use single also can, only pick-up quite slow because the computer tells your engine it is still a SOHC version)
Belting - DOHC is longer than SOHC, so must use DOHC type ..
Throttle Body - the injector needs to be bigger for DOHC ..not sure if you just change the injector can or not ..
Cable Plug - 1 set with the 'house' (i cannot remember when i write this, the one have contact point inside .. lol)
anyhow, that what i was told ..
about the leaking, just change the gasket block, ok liao .. (that is what he said to me ..)
so if any sifu out there can correct me or confirm this, we will be greatful .. thanks ..