If you can provide me with the exact model you have there I may be able to assist your further, however I would have a great deal of concern in if the parts were available converting an appliance of such an age as a number of different things have to be disturbed to be adjusted and I would strongly suspect that following conversion it would not be either as efficent or reliable as it has been in the past 25 years of use.
Most manufacturers only design water heaters ( of all types ) to have a life span of around 10 – 12 years, in your case you have more than doubled that by good luck. I would strongly suggest you look at purchasing a new unit which should resolve all your problems and give you reliable operation on your new fuel gas.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians