http://libindx.moray.gov.uk/mainmenu.asp
he is on there but birth date 1730 does not say very much, no parents listed.
That birth date has been 'calculated' by subtracting the age on the gravestone (50) from the year of death (1780). The stone is the only source of information about this Alexander Riach (and about his wife and possible daughter-in-law) in LIBINDX.
As he might have died before his birthday in 1780, he could have been born in either 1729 or 1730, assuming his age at death is accurate.
Where did you get 1729 from, Norman?
Golden rule: never believe anything you find online unless it is an original document. Use it as a finding aid, not as a definite fact. I see that the International Genealogical Index has two children of Alexander Riach and Elizabeth Bremner: Peter b 1771 and Janet b 1776. However the earlier Rothes baptisms tend to omit the name of the mother, so Peter and Janet could be just the youngest of a larger family.
In your place I would want to have a very good look at the Rothes parish register. I'd extract all the references relating to Riachs, and try to sort them into families using the additional information in the original, including the places of residence and the names and (where stated) relationships of witnesses. Rothes, unusually, often says of a witness 'grandfather to the said child' or 'grandmother to the said child' and I have been able to sort out some family links using this information. Also note any Riachs who witnessed the baptisms of other people's children, and any relationships mentioned. I found the name of a 5th-great-grandmother that way.
Are you anywhere near a library that might have a copy of the Rothes parish register on microfilm (Inverness, Elgin, Aberdeen etc)? Or anywhere within reach of a Scotland's People Centre (Edinburgh, Glasgow etc)? If not, you could view all the Riach listings in the Rothes parish register at
www.scotlandspeople.gov.uk - but there are 271 of these and it would cost you a fair bit. A much cheaper way would be to go to your nearest LDS Church Family History Library and ask them to rent the film for you for a small fee. You can then go through it at your leisure and see what emerges.
I very much doubt that there is any other way to find out more about your Alexander Riach, and there is certainly no easy way.