The complete Stella tail light unit can be swapped out for the Bajaj Chetak tail light unit. New holes must be drilled in the body and a rubber gasket can be built up to fit it properly.
When the Bajaj Chetak H4 headlight lens has cracked, some riders have replaced the whole unit with a car sealed beam unit like the Volvo 240's: GE model GELHE54697
The Bajaj lightbulb for the Chetak is unique (wider tabs) but a H4-35W
or H4-55W or H4-60W will fit.