My question is whether or not you can modify the heads sufficiently to flow enough to support the needs of the larger motor??
On A-series minis, you can get the 1275s out to 1600cc with offset (and huge) overbore and offset ground crank, but they only end up making the same power (with only very very very marginally more midrange torque) as 1400 odd cc ones which are much rev happier, and last a lot longer (and cost less).

It would be a huge ask to try and add deck height to a cast iron block, you'd have to have it nearly glowing red before you start to weld and let it cool in a barbecue or something with lots of hot coals to let the temp die down slowly, lest it crack