Same above here... just I'd go for a 2 stroke engine instead... I don't intend starting an issue on this... The reasons I think a 2 stroke would be better for you are:
1- lighter and powerfuler than equal sized 4st.
2- Easier to carburate properly
3- Less maintenance.
4- Faster idle-full throttle transition speed

I think these simple reasons would make a 2 st. much more reliable for TRAINING PURPOSES. FWIW.
Best of luck with your choice.