It may appear that some lift chair models are so old that handheld remotes are no longer available. However with some minor modifications, there is a remote that will work with the lift chair.
The motor control box must be removed from the lift chair with replacement of the hand control. A Pride document is attached the includes the instructions. In most cases, someone with reasonable mechanical skills and few if any tools will be able to perform these changes.
IMPORTANT Performing these modifications requires that the chair be in the raised position. So the existing remote must be available and operable to do that. If the existing remote or other issue prevents the chair from being raised, the chair will require a service technician.