To answer the original quesation..
If you can get rings and gaskets, and pistons are not badly worn, you can probably reuse what you have..
Check the parts carefully, , some bits like springs should be replaced.
Not everyone need the lastest, lightest , most modern parts..
Think about what you are using it for..
Some of us got 25 years or more out of a big-bore CB750, and take them on road trips.
Build what suits your needs..
They are vintage bikes, and are usually worth more with period correct parts, nothing wrong with a accurate example of a 70,s bike..