Depends on what you are wanting. 2 cycle or 4 cycle. In frame mount or rack mount. Front wheel or rear wheel drive. Too many variables out there.
From my experience, any offered warranty is a joke......you just hope you get a good engine to start with. Most of the kits & parts offered originate from the same source with varied pricing. There is some good improved after market stuff available from various venders. Some respond better than others.
Good luck with your project. Each one is a learning process.
what about the honda GXH50? anyone have any experience with it?