Getting back further than mid 1700s in Scotland is tricky, as so many people just weren't recorded and for those that were there's a lack of detail. You also need to flexible with names as spelling wasn't standardised back then so it depended on how the parish clerk chose to write someone's name. Muir and Mure will just be spelling variations of the same name.
When searching the OPRs it can be a good idea to look at the free site FamilySearch to narrow down the field before going onto ScotlandsPeople. A quick look on FamilySearch show the following that could be for your Muir family.
Margaret Muir born and baptised April 1750 at Carnwath, parents John Muir and Janet Muir
Mary Muir born and baptised March 1753 at Carnwath, parents John Muir and Janet Muir
John Muir born and baptised March 1755 at Carnwath, parents John Muir and Janet Muir
But, oddly, I couldn't find the baptisms for Janet 1745 and Agnes 1746 on FamilySearch. And there's no marriage for a John Muir to a Janet at about the right time.
There's a John Muir born November 1714 at Carnwath, parents John Muir and Agnes Somervell. Given that your John Muir called his second daughter Agnes and possibly his eldest son John, this tallies with use of the Scottish naming pattern. But that isn't enough to say with certainly that 1714 John is your John.
