This week I measured the wall thickness of a 1.5 84mm bore block and a 1.7 87mm block by removing the freeze plugs. Wall thickness of the 84mm bore was 5.5-5.9 mm, from the 87mm bore block also around 5.5 mm. The castings are different! Measuring the depth from the freeze plug hole to the cylinder outer side is greater in the 84mm block, this explains the same wall thickness despite different bores. Now, boring the 84mm to 87mm would reduce the wall thickness from 5.5 to 4 mm. A problem? Probably not in everyday use, it's the same as boring a 87mm block to 88,5. Some say 88 is maximum, some bored to 89 mm for racing and the block didn't last very long.
Boring a block here in the Netherlands costs 45 euro per hole, used 1700 blocks are cheaper.
Then there is the problem, you seem to have the 1600 with 84 mm bore from the 1500 and 72mm crank from the 1700. Pistons from the 1700 won't be compatible! Compared to the 84x67mm 1500 the 87x72 1700 has same length conrods and to compensate for the extra stroke the 1700 has the compression chamber in the head. The 1500 has a flat head with compression chamber in the piston. If the 1600 has flat heads and pistons with relocated piston pin, then you'll also need 1700 heads, 1700 pistons will give way to high compression ratio.
All in all, best to buy a good 1700 block, they were cheap here for a long while but getting rarer these days.