keep Richard Cromwell from taking power and you're well on your way, I've heard a few time one of Oliver Cromwell's older son was going to take power but died too young leaving the job to Richard
Tes, Oliver Cromwells son Oliver Cromwell was generally regarded as being a lot like his father though he died as a captain during the First Civil War.
I generally think you've got to either have Oliver Cromwell Jr
or Fleetwood take over from Cromwell Sr. However if you have Henry Ireton survive again he's a prime candidate (less preferable than Oliver Jr but by-far more preferable than Fleetwood).
If Cromwell Jr survives IMO (and in a TL I shall be reviving) England will become a Kingdom again under the House of Cromwell, if Fleetwood takes over it's likely England will still be ruled with an iron grip without a secure Parliament while if Ireton takes over it's likely we'll see some kind of elected monarchy come about as Ireton was a Monarchist I doubt he'd have felt comfortable having his family-line take the throne.
Just some food for thought.