We haven’t quite got a working fusion reactor yet. One that is autonomous that we can put is space I think is a few decades off.
If we are talking about fission reactors (all that is available today) we are talking about an expensive piece of hardware and if you include autonomous robot reactor, we’re talking big money.
The difficult thing about fission reactors is that after a fairly short time at power the reactor will continue to produce a prodigious amount of heat even when shutdown.
If that heat can not be removed from the reactor core the core will destroy itself.
The robot reactor you’re talking about sounds like a multi-billion dollar machine. I don’t think we want that bit of tech out there on its own with no one to take care of it.
Photon-intermediate direct energy conversion - works with fusion or fission.
A reactor in space can be very simple. No need for all the shielding. No need for all the safety stuff. In other words dirt cheap. Trail the reactor behind the asteroid on a tether with electrically melted water flowing back into the reactor to act as cooling water and pipe the steam back up to the asteroid to propulsion nozzles. That way we dont end up with an enormous chunk of radioactive ice.
Hell, drop a a bunch of water and gas asteroids on Mars and start terraforming it. It took a long time (billions of years) for Mars to lose its atmosphere the first time.