There are a lot of derivitives of the o-235 that work. I believe that there is an "L" series that is the one that really works the best. Although people have put larger o-320 engines in them, the additional weight makes for a zippy final approach speed and hence, a long runway is necessary.
I'm sure you've found the websites that cater to lancair enthusiasts. There is a good resource for you. I would caution you though that a lot of thought was put into what engine to power that plane in the beginning. If you can find a rotary substitute (and for that, I would direct you to Atkins Rotatry Power at Thun Field, Puyallup, WA- speak with Dave Atkins) great. But the aircraft is in the experimental category. You don't want it to be too experimental or you'll spend a lot of time in the hangar.
On the O-235-L series, the variant that is used on the Cessna 152 didn't have an accellerator pump on the carburetor, and with a 28 volt starter, the engine spun over too fast for the impulse couplings on the mags to catch. This results in a cold start difficulty. The version in the Beech Skipper and Piper Tomahawk did have a pump however. The 152 also only had a primer nozzle on cylinder number 4, and that contributed to the start problems.
The carb also tended to run too rich on the ground, which would foul out the plugs in short order. Leaning aggressively on the ground will save the plugs and also running the REB-38E spark plugs with the extended electrodes help. This is true with all versions of the O-235.
The -L's also don't have hydraulic lifters, they are adjustable, and I would encourage checking them at 400 hour intervals. There is a Lycoming service release for adjusting the valves, which includes preloading the cam for the adjustment. One good thing is the 2400 hour TBO on the engine.