Welcome, Jim.
You will get differing answers to that question. Mine is "not much beyond the integrity of the engine itself".

Installation of a rotary in a Cozy is fairly tight and, by the way, in my opinion a 20b is a little too big, long and heavy. Apart from test running the engine to be sure that it's been correctly rebuilt, most of what you do on a test stand will be a waste of time and effort. When installed on the plane the engine is dependent on many systems that either can't be effectively duplicated on a test stand, or will have to be re-engineered when installed in the plane.  Sure, if you can get your hands on a dyno you can test power and tune, but once you put the engine on the plane you'll have to wire the engine into the plane's systems and implement fuel, ignition, injection, cooling and exhaust systems that fit in the cowl. Testing those systems in the "real world" is really only possible with air flowing through (and over) the cowl into a prop at 200 mph and at various attitudes.

A normally aspired Renesis will work well, but I think will need very careful attention to intake and exhaust optimization to get the power you're probably hoping for. Next you will need some sort of muffler that can hold up. I wasn't keen on hanging a muffler below the fuselage like the RV guys do. There isn't much room for that in a Cozy and it would look very ugly and draggy. Putting a red hot muffler inside the cowl is a heat radiation problem, plus there's very little room. A turbo is relatively small, circular and easily screened for heat. My approach was that the easiest way to silence the roar was to install a turbo. This way you don't have to bother with an exhaust system, and the intake system is much less critical. This gives added (and exciting) benefits in take-off and cruise power, but comes with its own complexity. The trick seems to be to use the right turbo. Then, if you use a turbo, the Renesis is a little high in compression ratio and you might be better with a 3rd generation 13B. I'm working with another builder right now on his Cozy project. We are duplicating my 13B exactly. That might be a useful indication of whether I consider it a success. The Cozy Girrrls are very knowledgeable on this rotary stuff,  and they also are installing a 3rd gen 13B with a big turbo.

In general, I'd say the rotary is more risky to start with - say the first 50 hours - because every installation is different and errors WILL occur. After the first 50 hours I think the rotary is well worth it. It's safer, faster, quieter, very much smoother, and much cheaper to maintain. The devil is in the peripherals. Redundancy is key. I have redundant fuel tanks, filters, pumps, injectors, plugs, ignition computers, batteries. Failure of any item is survivable because I have a spare. The problem is that redundancy breeds complexity - this is where the extra build and testing time comes from. I'd recommend the IVO in-flight adjustable prop.

I've seen too many failed rebuilds by people who have much more experience and talent than I, to risk applying my skills to this most critical part. Also, there's enough to do with the peripherals and I didn't need the learning curve. Unless you're determined to rebuild your own you might consider using an expert who's done it 1000 times and knows what to modify for aviation build your engine. Given your investment in the plane, a professionally rebuilt engine is cheap insurance. Instead, I'd suggest concentrating your effort on getting the peripherals perfect, adding redundancy everywhere you can get it.

Having said all this, my take, for what its worth, would be to forget the engine for a while and spend every spare waking moment building yourself an airframe. Given that you have a few years to make the engine decision, use those years to see what happens with other installations.

I hope this helps in your decision process, or at least starts yet another heated discussion you can learn from :).
Regards,
John Slade
Turbo Rotary Cozy, N96PM.
98 hours and purring like a kitten.