For the 1960's era 9-bolt Triumph 650 engines their is a variety of aftermarket specialty cylinders to increase the bore size, reduce weight or both. Some of these would be the Routt 750, 800 and 850 jugs, the 750 alloy Zip Kit, the Morgo and MAP delta shaped cylinders and my favorite the alloy Chantland 750 and 800 kits. But for the old pre-1963 8-bolt cylinders there never was a big bore kit due to the bolt pattern being too close to the cylinder bores.
The one and only specialty cylinders I know of that were made for the old 8-bolt engines is the Wellworthy Alfin Cylinder Barrel. These were a finely made peice that was based on a steel center that included the head bolt mounting area and cylinders as one peice with the alloy fins and base flange being cast directly to the steel center part. All the other later alloy cylinders had pressed in steel sleeves that could come loose and also making for weak head bolt threads. The ALFIN cylinders have slightly larger fins than a stock cylinder and the cylinders on the example pictured below had a larger outside diameter cylinder spigots making these useable to a .080" or .100" overbore making them something of a big bore kit as well. If anyone has one of these that is not broken I would very much be interested in buying.