Do some motorcycles have only a rectifier?

Not as far as I know. Unless we're talking about some sort of weird 100-years old design, some sort of regulator is present on any motorcycle since they tend produce greatly varying voltages at different speeds, and using that to charge the battery is Really Not Good for it's lifespan. However, many bikes might only have one "box" in the circuit, which integrates the rectifier and the regulator together. On my 350 Jawa, those were two separate units. On my newer Virago 535 they are a single unit. Therefore, if you only have one component between your generator and battery which is outputting un-stabilized voltage, one of two things is likely: either the bike used to have a regulator in the past which got "lost" somehow or your single box does include a regulator but it's fried and needs to be replaced.

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