Aint broke don't fix it 65/70 is good for a DT. By all means pull it to bits if you want but i doubt you will get any more ooomph, maybe less as the bike could be in the sweet spot before the bore finally wears out.
One point to remember though, these engines rarely get past around 15000 without work, 21000 on the original pot is unlikely. Not impossible, not that common either. Unless he has owned it from new, i'd make my own decision on the top end. Keep your money in your pocket till it needs it, you will know soon enough .