I'm a little late to comment on this as it looks like this string has all
but died. I measured a spare block tonight and it looks to me that at
their thinnest the cylinder walls are about .185 thick. If you overbored
all the cylinders .090 you would end up with cylinder walls about .090 plus
or minus thick and about 1600 cc. So it might be possible, but why? You
gain about 6 cubic inches, weaken your block and lead to further
overheating problems. Assuming the engine is already tuned to give 1 hp
per cubic inch or slightly more, you pick up a few horsepower at the cost
of major expense and in my opinion a shorter lived engine. I think I'd
rather invest in building or adapting a crossflow cyl. head.
Just my thoughts, a few days late.