I've checked the Ayrshire Burial Index and there are only 2 Alexander Allans listed and 10 Janet Campbells (mostly children). No links to Galston Parish.
There are several problems with Burial records from that time period
1. They were poorly recorded
2. Many were not recorded at all except for records of fees paid to hire a Mortcloth
3. Many records were poorly stored and didn't survive
4. The schisms and rejoinings of the various church congregations tended to scatter any records.
5 and some were just lost

I've just checked the official list of OPR Records from Scotlands People and for Galston the only death records known are for 1762 - 1763