As ever without looking and measuring you can't tell. However from experience I would say that your engine needs a re-bore. Usually with those symptoms what has happened on a B20 is that you have a broken ring or two. Likelyhood is it will have scored the bore and no amount of re-ringing will cure that.

The pistons are excellent quality, if you find a late B18 engine block you could have that bored to 2 litre, re use the B20 pistons with new rings. Drop straight into your car no mods, B20 head will go straight onto it. Usually if you can find one they are pretty cheap , at least here.

Just make sure you get one with the mounts for your alternator or you will be fabricating, '67 on should do it. The other big but is that it is only easy if your B20 is a six bolt crank engine ie 6 bolts holdng the flywheel to the crank, the later 8 bolt crank is metric and the pistons won't readily fit the earlier block

Regards