New rings might fix it but you might also need a new piston and also need to check and measure the cylinder for size, condition and roundness. But your diagnosis of a new top end is probably correct. It could be as simple as a new head gasket or a loose spark plug, but I doubt it. On the rear, you can maybe adjust the preload on the spring to a stiffen it to a certain degree (I'm not familiar with '88 YZ80 shocks--usually a ring on top of spring that can be spun to compress or decompress load on the spring), but more than likely also need to rebuild and charge the shock. You can buy a new one or have yours rebuilt. Could be as simple as a seal, fresh fluid and nitrogen charge. But it could be as bad as a worn shaft, requiring more work or replacement. Check around at shops or on the internet for shock rebuild prices or replacement shock. Good luck.