It seems to me that you are not very confident about your abilities, therefore I wouldn't recommend trying to solve the issue by yourself. Cosworth engines are known for their reliability and ability to generate power and high performance. Most of the time, an engine starts spewing oil because of degraded engine gaskets, oil pan leaks, oil seals or bad connections. Since cosworth engine internal components are highly engineered, it is slightly harder to get all the accesories you will need, compared to bmw accesories, for example, and to swap everything perfectly. I wish you good luck in your attempt and let us know about the result.