we have a very old house (1845) and the original hardwood floors have gaps/are cracked and need replaced, is there a problem putting new hardwood floors ontop of the old? any suggestions

the floors have been damaged because the house is shifting under them. logic would suggest that the shifting be tamed before reflooring.

I would start with a level line (showing my age, a spinning laser level is a better tool) to check the joists underneath. if there is central sagging, find out why, and fix it. if the floors are under-supported, you may need to sister more boards against the joists after levelling up. screws and glue.

at that point, I would strip the damaged flooring, check the floor for trueness with a long level, and put new subflooring down. then put new hardwood, or as much of the old as you can salvage, with some new boards or reuse center boards in there salted in randomly so they don't show so much. then refinish.

I strongly suspect you will find sagging in the center, and perhaps deterioration of a center support as the main reason for this. but shifting foundations or other drooping could pull on the floor. if this is on a wood or worse, a limestone foundation, it could turn into a real major project. there is a lot of that on Dayton's Bluff over the Mississippi. limestone blocks eat the wood framing where they meet. and you touch the blocks and they crumble away.

Under sized joist with to long a span.
Spacing of 24" or more.
Insect infestaion.
Fungus damage.
Cracked joist.
No center beam running down the middle of the spans to support the floors.
All the lovly things we find when working on older homes.

Best way is to rip up the old floor. If you want to hardwood floor on top of the existing one its difficult to keep all the floors the same height without transitions.