Funny story related to this post... We once thought that all flat head continental engine blocks were the same and that they could be simply bored out to accept the next larger displacement pistons. With this flawed knowledge we hogged out an engine a full 1/8" (or was it more?... I can't recall). The engine was assembled and fitted to a small bulldozer. It fired up, ran well and the owner marvelled at its new performance. A short time later, on a particularly hard push, one of the cylinder walls of our "big bore" engine ruptured, admitting coolant at a high rate of speed into the combustion chamber. The coolant blew out of the exhaust stack of the still-running engine, showering the poor owner as he scrambled for cover. Those in attendance remarked that it looked something like old faithful. They also noted that the owner, a very large man, had never been observed to move with such haste...
The owner was fine, if less than amused. Disassembly of the the engine revealed a quarter sized hole in one of the cylinder walls. The cylinder wall was less than 1/8" thick at the point of the failure. Keep in mind this was a very mildly tuned gasser with pretty moderate cylinder pressures...
The 1.6TD (76.5mm) block would require a 3mm overbore to take 1.9TD (79.5mm) pistons. Thats about what we took out of the aforementioned continental to fatally weaken it. I would be tempted to take a junk block and have it bandsaw it in half or drill a hole in the cylinder wall to determine the thickness. I wouldn't bore it if the wall thickness would end up less than 3/16", you need to leave a little safety margin for "core shift", corrosion and other variables.