The Veron Cardinal has been to date my most susesfull free flight plane.
Use the right metals for the engine B&Q alluminium is not a good idea.( Been there bent it chucked it)
Bicylce alluminium pedal cranks are worth a go.
Best place would be a metal scrap yard, you will probably find some old truck pistons that can be melted down.
There's loads of plans for engines all over the internet, and plenty of books to.
Good luck