Strongly suggest finding and installing the correct factory part and keep the receipt.
The ROPS is a (now required) safety item. As we all know, there are stickers on it which warn us not to drill holes in it, weld on it, etc., because the function of the item might be compromised. Even though enlarging the hole by 1mm for a larger pin isn't very likely to make the ROPS utterly collapse from the strain of a hummingbird landing on it, we have been warned by the tractor maker's legal department not to modify the part.
I ran this discussion thread past my Dear Bride (tm) who has been an attorney for over 30 years.
In her opinion, if anything happens (someone hurt or killed) the insurance company will examine the ROPS and the claim will be promptly denied because "plaintiff performed unauthorized modifications to safety equipment". Even though you, I, and any competent machinist KNOW that the "unauthorized modification" (enlarge that hole by 1mm) wouldn't make a darn bit of difference, the insurance company and the manufacturer do have that little label to hide behind and will be successfully able to say "we warned you not to do this, it isn't our problem any more."
Same thing for a DIY bolt, they could claim it might be too hard, too soft, not correct fit, wrong color (huh?), whatever, "unauthorized modification, and we ain't paying."
When comparing the real world and legal world, as they say in the movies, "any resemblance is purely coincidental".
Chances are nothing untoward will ever happen, but the minor aggravation today of finding the correct part will keep you from ever being the subject of the old curse "May your life be full of lawyers". (It may anyway, but at least not over this.)
BTW, I now get to buy her lunch for this tidbit of free legal advice. Or we may take it out in trade.
Best Regards,
Mike/Florida