The hassle is that every foundry that makes CI only has to standardize on the ID of the main pipe...the size and shape of the hub can, and does vary. One of the major makers of the 'donuts' is Fernco, and if you go to their website, you'll find LOTS of different versions to account for those different sized hubs. The local supply shop may not have the one you need. Too big, it will compress the pvc pipe too much, and it won't likely be able to be inserted with the pipe...too small, and it will leak. Just like Goldilocks...it has to be just right. Modern PVC/ABS, etc. IS pretty standard, so the ID's sealing surface to the new pipe is fine, it's just getting it the right thickness to allow insertion and enough compression to seal well.
With the right tool, inserting the pvc into the leaded joint, then compressing it with the calking tool, MIGHT make it seal, but it won't be as tight as a properly fitted donut. Depends on the state of the oakum beneath the lead.