From the info you give, you could have mostly anything. Since you're renovating the bathroom you can thus DO mostly anything.

Is this an open loop system (do you need to vent steam)? You can replace like with like. You can get something fancy instead. You can use any old finned tubing. You can hit a demolition/recycling yard. You can hang a heated towel rack radiator. You can lay down steam sub-floor heat. You can lay down electric sub-floor heat.

Sounds like more of an opportunity than a problem! There's no real restriction.