Go to - there is a guy on there from Hawaii that is using a 4ac on twin carbs. From memory he has a ridiculously rare HKS twin sidedraught manifold.
At the end of the day though the 4ac is a non cross flow head so it's never going to flow as well as a 4age. I have heard guys in America getting close to the 100kw mark at the flywheel but your looking at a pretty grumpy, heavily worked motor that will just keep up with a stock 4age (maybe, I would doubt it would even manage that due to the torque deficit it would more than likely have)
Also you will find that unlike the g series motors in the old Gemini's no one over here has really spent time/money on a 4ac. Dave (Truenosedan on here) turbo charged one but it was a bit rough and a bit of a hand grenade (no disrespect to Dave, I'm paraphrasing his own words here)
If you wanted to go completely silly with it you could always use a 7afe bottom end for a 1.8 litre engine but I don't think it's been done before. Another option is to use a 4afc head/carb - that is a narrow angle twin cam head so you would pick up a few kw - also you could probably pick up a whole car containing a 4af (ae92 Corolla from memory) for the price of a 4age and have the rest of the car to wreck and sell...
I personally am going twin carbed 4age - manifolds are available off the shelf and it has been reasonably popular over the years - I won't revisit the ins and outs here, a quick search will answer any questions you have.
Sorry this was so long winded, I just wanted to get in before 50 people posted saying "sif do up 4ac, " etc etc