oil hose

1/2 Inch Hose will be fine as long as it seals onto your fittings and is stiff enough to not collapse.
The supply hose is under a slight vacuum and the Return Hose is under low pressure (<10psi).

Personally, I wouldn't risk a $20,000 engine on a critical part to save $50. Buy either aircraft-approved or, from an established Rotax supplier, the recommended hose and fire sleeve.