I've looked at that, they are much more rare here, a least the 5 valve version. In my case I was interested in something that might be smaller as well as lighter. Reducing the weight back there seems it would a worthwhile pursuit. The G13B can produce plenty of power with ease and would start out with considerably more power than the 138 series motor. It is also good looking motor which would add to the engine bay.
Which does of course beg the question, why not just mod the existing motor. It fits, it is a fine motor and would maintain the existing balance of the car, weight wise.
One of the things I like about the G13B is that I could keep the emissions systems and get good fuel economy. The car it comes from is about the same weight as an X and has more frontal area so my economy would likely remain the same as what the Fiat motor can deliver (in the 30s which has always been a great part of owning this car). In any case, keep the commentary coming.