You can just replace it with wood if you like. I would suggest ash or maple. Oak has open pores and will weep. It won't get hot enough to burn, and the wood will conform and you won't need a gasket.
Otherwise you could buy new phenolic and mill it with a router. Phenolic material has the cloth reinforcement but uses phenolic resin instead of the bakelite resin. Its like fiberglass but a lot more sturdy. Woodworkers use phenolic sheets for tool table tops and router bases, so ask at a well stocked woodworker's supply store for small peices of stock.
