A 1700 T4 (in decent condition) works fine on a 6 rib, even pushing a Bus, BTDT.
There's much than the obvious ~100cc displacement between a 1.7L T4 and a 1600DP going on.
Your jugs and/or rings are probably worn out, do compression/leakdown testing as suggested.
The bottom end may be in great shape, the rods can be pulled to inspect the bearings once the pistons are off.
If the rod bearings are good, and the cam/lifters don't have any obvious issues, I'd stop digging, unless you plan on a full rebuild.
Even in that case I'd treat the main bearings carefully as the replacments are expensive and may not fit as well.
The heads should probably be looked at carefully by a t4 specialist.
"The nice thing about standards is there are so may to pick from "