Mine has c3j on the plate on the block, 760 code iirc so 1390cc, but the block is stamped c3g. I would suspect all blocks are very similar/same as c1j.
Gtt pistons and liners went straight in, and with all new bearings, rebuilt gtt oil pump/sprung campus tensioner, the oil pressure is better than any previous c1j’s. 55-60lbs hot at 3k.
Ive not seen the lower cc blocks before, is the crank the same as the rest of the models?
Quite a few bits are needed for the conversion, as above gtt oil pump, gtt pistons and liners, gtt rocker bar, gtt head if using stock ignition and a couple of small other bits. If you dont have tons of spares kicking about, it will be easier to pick up a c1j and stick it in.