Welcome, Cliff - Happy Birthday and welcome to the Club!
As for you specific question - unless you already have an STC in had to perform such a conversion, you'll be hard-pressed to get an O-300 approved for use in a Cessna 150. Not impossible, but I'd sooner buy waterfront property in Arizona and wait for global warming....
The only 'big-engine' conversions use the Lycoming O-320 and O-360. Even with the STC, the cost is so prohibitive that you're far better off buying a flying example than doing the conversion yourself. Many members own and/or have converted such planes, so I'm sure they'll pipe up with specific advice.
