Hi Rin,
Schooling is definitely different in Hungary, students here in the UK won't be back at their desks for another week. From what I remember there are a number of straight forward oscillating engines in Elmer's drawings. I have not made any of his engines however, if you haven't looked already looked in
https://www.homemodelenginemachinist.com/ there is often a build thread of Elmer's engines and you might get answers from someone who has built what you are considering. Just had a look on the site and found this
https://www.homemodelenginemachinist.com/threads/not-bad-for-a-college-student.34303/ not an Elmer but could prove useful.
Good luck with your project.
Take care Tony.