How much is a 1964 Mustang without a motor worth ?
If it's free and you have the storage space, by all means take it.
There is a variety of engines you can fit into that bay (289, 302, 351, etc...). And they are very plentiful, with literally thousands of options as far as parts as well. You'll never have any issues finding parts for early Mustangs.
Even if you need to sell it later on, know that someone will want it.