You fail to say whether your engines are steam or IC , generally in steam engines of around 1/2" to 3/4" bore and low superheat I would recommend viton o'rings , I use the "quadra lip " variety and have found them to be very reliable.
The fitting tolerances quoted by the manufacturer are for high pressure hydraulic systems and should be relaxed for model engines or they will be very tight. Silicone o'rings should not be used with steam.
For bores greater than 3/4" cast iron rings are generally prefered , high superheat temperatures will destroy most o'rings , viton can be used at temperatures up to 180°C.