The SK .50 engine is imported and distributed in North America by www.kangkeusa.com. They're not widely distributed, but many good brands of engine are not. Engines and parts are available directly from the distributor; there are also resellers like:

http://www.bj-model-engines.com/home.html

...who sell SK engines, service SK engines, and provide parts for SK engines.

Similar to other brands of engines like Magnum, Super Tigre, or Evolution it seems that most owners are quite happy with their SK engines while a few users may get finicky engines that they constantly struggle to re-tune.

I own one of KangkeUSA's JBA .56 engines, and it's been terrific so far.

If you or your flight instructor is comfortable with tuning engines, you can save yourself a chunk of change by trying the SK .50 2-stroke. It's a solid value and KangkeUSA and it's resellers will provide service and support.

If you aren't comfortable with tuning a 2-stroke engine, spending extra money on a Thunder Tiger Pro .46 might be a good investment.

Good luck and good shopping!