I don't see the dia as a safety issue, as many street bikes have 14's and go over 110mph, which is faster than many Harleys will go. And of course side cars can run 10" and 5" rims.
There really is no safety problem with a 14" tire EXCEPT, the highest DOT speed-rated 14's are like 118mph@600lb but are not recognized by the SCTA. So you can only go 70mph on them. They don't recognize the modern designations; L (75mph), P (93), S (112), T (118), (V) (149+), W (168) or (W) (168+). Which is odd, since the W's, (W's) and (V)'s are the high speed DOT tires, not H/V/ZR. The ZR (150+) rating is only valid if you aren't running at full load. It's an obsolete rating that is on the books because of equipment laws in some countries. Modern tire designations are a number and a letter. ZR is included also sometimes, but isn't the speed rating, it's a legal insignia. The tires on my car say ZR (150mph+ at under the rated weight) but they also say (96Y) which is sustained speeds of over 186mph. Sadly, the dumb thing came with a factory speed limiter of 175mph. Wifey won't let me reprogram it.
But rules are rules. If you want to go over 70, you need to first find something that says H, V, ZR on it. If it just has the modern designator, it might cause you trouble.