I would lift the cylinders to have a look. If no diagnostic tests for compression or leak-down has been done previous there is no results to compare with. Tests can give a hint but expect the worst. It can be as easy as just ring tension or, I am not saying it is, but in worst case it can be the ever recurring problem with Indian engines, Piston pin circlips that jumps out of the groove.

Of course you might find nothing obvious. Modern low friction oils can prevent rings from seat properly, glazed (shiny, overly smooth surface) bore might be difficult to diagnose, or on the other end, piston rings can lose tension or wear down quickly and that's bad but does not take a lot to fix. Check ring tension and end gap and perhaps put new rings in. Brush the bores just a few strokes with a suitable flex hone. 

Broken rings on the other hand in most all cases needs new pistons because the precision cut ring lands are, and should be regarded ruined. Bore measuring with a bore gauge should be done but perhaps just a honing job with rigid stones is needed to save cylinder linings. What is worse, and very expensive breakdown is the more common fault with the Indian engine, I repeat, Piston pin circlips that jumps out of the pin groove.

In any case whatever is found wrong or right in the engine, I have to recommend use the safest piston pin locking system that I know of, bar none. Machine square circlip grooves in order to fit dual spiralox retaining circlips on each side of the pins. With a proper special tool, mounting and demounting spiralox is not that of a big hassle.  

No problem to demount or mount the cylinders in the frame even with the tank in place. 

To mount the cylinders much, much easier I made a piston ring compressor out of a 3" diam. 45mm tall ring of ordinary PVC sewer pipe. The ring is slit, and has a pair of micky-mouse ears riveted on in order to be able to compress the piston rings with just 2 finger pressure. The thickness and height of the pvc tube is really advantageous as -resting on the crankcase- it serves as a support for both the piston and cylinder. 

When the piston is at bottom dead center, the compressor ring is just as tall as it covers the piston rings, and the piston top that is sticking up above the ring is serving as a guide for the cylinder. The thickness of the ring steadies the piston, and makes it possible to balance the cylinder on top of it with one hand, squeeze the ring together to compress the piston rings with the other hand, and kick the rear tire (gearbox in gear) to rise the piston into the cylinder. It sounds elaborate but is really very easy technique and much safer doing it single handed! than any other way of doing it that I know of!

PVC is easy to shape with a hot air gun. I cut the slit and shaped the ring slightly smaller than the piston diameter so it stays on the piston without effort.