With only 90M miles and since it isn't driven hard you should have good compression so it probably isn't the rings
but here it goes...
There really is no easy way to replace the rings, you will have to pull the motor, pull the whole top half off, intakes, heads, etc.. pull the oil pan, pickup tube and pump, pull off all the main caps, the main bearings, then you can unbolt the rods from the crank, pull the rod bearings out, pull the crank, then you will be able to drop/push the pistons out through the bottom of the bore, put some plastic or rubber tubing on the end of the rod bolts so as you are pushing the piston out if it makes contact with the crank it will not scratch or mark the crank. If the crank shows marks on the bearing journals it will need to be fixed, polished etc... Whew, man I was on a roll.... Then you can take the old rings off, all three on each piston and clean out the rings valleys/grooves after these are all clean you can put the rings back in, all the ring gaps must be 90degrees from each other so that it hold compression and doesn't get massive blow bye. Put all the pistons back in the with the top ring gap facing the same direction each time, you will need a ring expander to put them on and a ring compressor to install the pistons. The pistons will go back in through the top. Damn forgot a step... If you mic the walls and the bores are within tolerance (I can't find my sheets right now to tell you what that is) then the bores will need to be honed to knock the glaze off, honing will put the cross hatches back on the cylinder walls so that the rings have something to seal against you need these. A machine shop can do that or you can do it at home if you are careful, the stones can be bought at Sears, the ball type are better then the flat pads though, the hatches should be on 30 degree angles. I did this with a 1/2" drive drill, very slowly and carefully to get the hatches back, the up and down motion puts these in, use liberal amounts of WD-40 to lubricate it very well. Wash out all the bores with soapy warm water to get all the debris out. Dry everything out lubricate it then install the pistons and new rod bearings, the crank, etc...
The valve guide is what the valves in your heads slide through, it keeps the valve straight during operation, each valve has a valve seal, these may be worn out, they have rubber and ceramic valve seals, it all depends on the application. A machine shop could will replace the worn out pieces or if you have the knowledge you could do it at home but you will need a valve spring compressor and other tools, even though I can do almost everything to my car and I am not afraid to try anything, I would let a experienced machine shop put these in, I would take them apart though, and put them back together. While your at it you could also get a valve job, port/polish, etc.. or make the decision to upgrade to a new set of heads, cam etc... the possiblities are endless once it's apart, it really all depends on how much you want to invest and what you want the car to do. Ask 89LXNB-Dan knows exactly what I am saying, his car is apart right now and it started out with a plan and now like me he is over budget and doing more and more..... it seems like it never stops. Sorry this was so long if you need anything else drop me a line, I am always happy to help
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My Ride Check Out My Site 91GT, Holley SysteMAX II Kit-Heads 2.02" x 1.60",Full MAC exhaust 1-5/8" Long tubes, 2-1/2" O.R H-Pipe, 2 1/2" cat back, 12" K&N Filter, RNH PERFORMANCE Ram Air, Ron Davis Radiator, Full Suspension,3:73's, Welds, Best on new motor-12.60 @ 108, no traction 1.98