I think if you want to go fast, and you can eat the rest of the solar system, you can probably make a huge swarm of fusion reactors to help blow matter off the sun. Let’s say you can build 10^11-watt reactors that work in space. Then you need about 10^15 of them to match the sun. If each is 10^6 kg, this is about 10^-4 of Mercury’s mass.
I think if you want to go fast, and you can eat the rest of the solar system, you can probably make a huge swarm of fusion reactors to help blow matter off the sun. Let’s say you can build 10^11-watt reactors that work in space. Then you need about 10^15 of them to match the sun. If each is 10^6 kg, this is about 10^-4 of Mercury’s mass.