I looked in the Onan manuals and it does not give the displacement. However the bore is 3-9/16" and stroke is 3". So that works out to 29.9 cubic inches per cylinder x 2 = 60 cubic inches or 983 cubic centimetres (CC's) so say 1,000CC's which is 1 litre.

Unless I calculated wrong, it's a bigger engine than I thought!

They show a Briggs & Stratton Vanguard 993cc kit, but it comes with an intake manifold. Their universal kits show up to 800cc. Since the Onan runs at a constant 1800 rpm, a kit for a smaller engine will likely work since we don't go WOT!

My Onan works fairly well, but it is smelly. I think a catalytic converter would clean up the smell, but they are costly.

Maybe someone will chime in with the cost of a kit.