you'll need to replace the head unit and speakers. no aftermarket radio can cleanly interface with the Bose system. In addition, the speakers are awful anyway. Note that back then the Bose amps were typically mounted to each speaker.
Any aftermarket speaker will be better than the Bose factory speakers.
JVC, Kenwood, and Alpine have inexpensive head units that are good build quality.
You can look on Pacific Accessory Corporation's website for any possible wiring adapters - they would use the RCA outputs from a head unit to feed as low-level signal to the Bose amps.