A lot of us have been using SLA batteries in our homebuilts since long before any mfgr went through FAA certification. The PC680/PC625 and their 'white label' equivalents work great. I've been using various white-label versions for a couple of decades.
Buying an Odyssey will guarantee you get a proper starting battery, but there are many other options if you know how to shop for them. Just don't buy the 1st one you see with the same AH rating, since many of them are designed for lower discharge rates & deeper discharge than a starting battery.
Charlie