I'm in SC so winters here are 15* at worst. It took my bug 6min to get to operating temp & producing good heat. That was running a 2.3L
Barney's, If I'm remembering correctly, they had an inhouse CNC for custom wheels! They made some good sturdy chassis too!
I usually got my stuff from Larry's Off Road. That was back in the day when Larry actually ran the place, and would bend over backwards to make a customer happy! 12-15yrs ago.
Back on topic!
I had a little stent doing small engine repair at work. Part of the job was testing fire equipment. FD runs 16HP B&S on the portable pump units. That's open air, and it takes them about 10-12min to heat up in 30* weather! I imagine the confined area on an argo would speed the process.
One hurdle is to find a low PSI check valve to place inthe oil cooler lines to prevent any problems of over fill oil level on start up! If the cooler is mounted above the fill level of the engine you would risk it draining back into the block when shut down. If you had a 1/2 + qt. over fill on initial start up, you could possibly do internal damage! That's worst case scenario, and more likely in severe cold weather.
I think this is along the lines of what you are looking for!
Perma-Cool 13215 - - summitracing.com