Our 1980 Cal 39 had a similar
holding tank. We replaced it with a 20+ gallon tank shaped to fit in the V-berth. You can get them pre-made. Just measure your space carefully.
The smells you have are probably hoses that have worn out. Replace them, and make straight runs. Heads are a consistent problem.
J. Williams
Spindrift Cal 39 (#105) 1980
San Francisco CA