Well if you can do the work yourself, I'd say go for it...
If there seems to be a bit of blue smoke around when accelerating, etc. then maybe the piston rings are past their best. You could also do a compression test - if you have (say) 140 or less PSI, then probably it would benefit from new rings.
Seven years ago I had an old Uno 60 which was very smoky. I think the engine had been overheated, which causes the rings to lose their springiness. I took the head off (as you have), took the sump off, unbolted the big-end bearings, and took the pistons/con-rods out. Then I gave the cylinders a quick hone with a three-stoned device (if that makes any sense
). Cleaned up the pistons with kerosene and kitchen scourer pads, fitted the new rings (cost: about ten pounds equivalent), and put it back together. An immense improvement (oil consumption dropped from 1L/500km to 1L/5000+km) and one which I would expect to last for quite some time (five years at least).
You could do the job properly by measuring the bore diameter, measuring ovality, etc.
What I suggest is to clean away carbon etc. from the top of the cylinder bore, and try to feel a ridge there. You often won't find one. If there is one big enough to catch your fingernail, then the engine probably is too worn for just a hone + new rings. I would get another engine (not worth the cost of rebore+new pistons).
Then take a new piston ring and push it squarely about 3cm down the bore using a piston. Measure the end gap with a feeler gauge. This should be about 0.30mm - less than this and you can file the ends of the ring gently (to increase the gap). Greater than this, and you have to wonder if the bore is perhaps too worn...
Replacing rings seems to be less common these days, I think because engines tend to be longer-lived and therefore easier to find a complete replacement for. In my opinion, rings are the first part of the engine to actually wear out, so I'd still replace them if an engine was showing the signs of wear (basically, blue smoke and the crankcase breather blocking up).
You are also supposed to replace all the big-end shells while you are there, but I just wouldn't do this: crankshaft bearings are extremely long-lived these days, and you risk more harm through incorrectly-fitted new parts in my opinion. Inspect to check for gouges (light scratches are probably OK, base metal showing through is a no-go). Cleanliness is essential at all stages of reassembly.
-Alex
