You can go for one of the K engines; 4K, 5K, or the 7K. They are 1300cc, 1500cc and 1800cc respectively. If you still want more power, get a 3S-FE. Still need more power? Get a 4A-GE.
But I would suggest you keep things simple. Get a carby 7K, it'll be enough for your power needs for now.