The major disadvantage I see to lightness with the 4A is that its an iron block. Yes, its compact and light and thin-cast, but its still iron.
Thats just plain not true - cast iron is no heavier than aluminium for blocks because of the required ribbing for aluminium for the strength.
This has been proven over and over and you need to look no further than the weight of many 4 cylinder Honda aluminium engines that hover over 200kgs. Audi famously with their 5 cylinder increased the weight by 25kgs when they went to all aluminium by the time it was strong enough. The famous '80's BMW F1 Turbo engine was standard based cast iron unit and Renault used a cast one for their qualifying (aluminium for the race).
If you want to make a special engine then go for something different, you ain't going to beat the big boys at their own game and your not going to beat 210+hp out of a 4AGE that some get so whats the point?
Something like rotary valve head or the fore mentioned cheap V8 - I would suggest 2 x K24 Honda VTEC heads etc. The other choice is one that a company is now making, a 'normal' inline block for the Hyabusa (name your Superbike) internals and head to take a normal car type gearbox even to further that a twin counter rotating crank Square 8 (perfect primary balance).
Hard to start a bespoke engine program in this day and age of manufacturers getting 100+hp per litre with a 3 year warranty to boot in a standard car (Honda S2000 for example).