It's important on a re-design to figure out why the current system isn't working correctly, or you could easily drop ten grand into it and end up pretty much where you are today. The problem may or may not be related to the B-W combi- it could easily be the pumping or the capacity of the heat exchange on the tubing. Hopefully it has extruded aluminum heat transfer plates between the tubing and the floor? If not, that alone might be the solution, but that depends on how difficult it is to retrofit them.
Every good heating design starts with a careful room-by-room heat load calculation, from which all else flows. A decent radiant designer would be able to zero in on the most cost-effective solution, but it's probably not something reliably resolvable in a "design-by-web-forum" approach.
At the current price of propane something with a condensing burner will pay for itself well within the anticipated lifecycle of the unit. But whether that burner is a customized combi based on something like an A.O.Smith Vertex or a Polaris water heater vs. a modulating condensing boiler operating on outdoor reset control depends on your water temp and BTU/hour requirements at your design-condition heat load. Calculate the heat load first, then based on your radiant floor design, the design-day peak water temps can be derived. Without those fundamentals as a starting point it's all just a big WAG, and the last thing you want to be doing is another round of "install first, design later", as it appears the original installation was. It's important to do the math before diving in. (And that's the REAL math, not 10 minutes of lipstick-on-mirror or crayon-on-napkin hackery.)
There is at least one competent MN radiant pro posting on this forum who may be able to take this on directly, or at least steer you to someone local to who can. (Paging Morgan Audetat...) Simply flipping through the yellow pages isn't gonna cut it.