Re-engine a norlet rotavator?

I have an aged but strongly made Norlet rotavator with a Briggs and Stratton engine which starts and runs fine but runs out of steam after half an hour or so and I suspect it could do with either an engine rebuild or replacement. I would prefer to fit a more powerful engine. Has anyone done this please?

Thanks for the response smud6ie, it's a norlett Bulldog with a 3hp B&S engine.