Hi there and welcome to the CAC!!
The value on early sharks is always a popular topic. There are a number of things which you will need to examine (or have someone examine for you) before you can get a decent estimate of value. The most expensive Vette will be a highly optioned, low mileage, all original big block roadster. Basically, you start with that standard and work your way down. The fact that the vehicle does not have it's original motor isn't as big a factor as it might seem. It's more important that the vehicle was originally a big block and you can tell that from the VIN.
Very few Vettes of the vintage you're discussing (and I presume you mean a 68 to 70... the early big blocks) have their original motors. They might have the original block (or not) but most have been modified, even if it was simply a rebuild. These are, after all, performance vehicles- most of them have been driven, driven hard and driven often. Motors wear out.
If you're looking for more specific value information, you'll need to share a whole bunch of details, maybe even post some pictures. Good luck and if you're really concerned, find a local CAC member who has the experience to give you a good recommmendation or pay a professional to evaluate the vehicle for you.
-Mac