Looks like a question about a Toyota motor snuck into a datsun forum covered with the mention of the A series.
We all know toyota mechanics, so its understandable
Years ago a friend's rollie had the same problem and we got a new chain and tensioner. We thought we were good and did it in the car. ( we realised later it would be way easier to just take the motor out)
Chain problem turned out to be a siezed tensioner that was easily fixed with some emery paper.
Both Toyota motors and the Datsun A series require the chain to stretch a friggin lot for the tensioner to become loose from stretch. The tensioners face also wear eventually.

Overall not a difficult job if we as young blokes could do it with just a workshop manual to guide us.