My thought is you left out the most important constraining variable: Year, make and model.
Via
duckduckgo.com/?q=652cc+air+cooled+rear+engine+-motorcycle the likely candidate is the Fiat Panda.
According to
www.classicandsportscar.com/features/square-space-fiat-panda-40:
Quote:
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At its launch, the Panda was primarily offered with two engine options: the 30 edition employed the air-cooled 652cc two-cylinder unit from the 126 Personal, with power boosted by a giddying 6bhp; the Panda 45, meanwhile, used the 903cc pushrod ‘four’ from the Fiat 127 that, remarkably enough, produced 45bhp.
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23hp sounds like not too much.