You should be able to date the engine by finding the words Model, Type and Code stamped into the engine cowling ( or in Briggs speak the blower housing). The numerals under. Code show the year, month and day of manufacture, so if they start with 55, the engine was made in 1955. If it’s s lot later than the 50s the machine may have been re- engined.
As far as I know the oil in the chain case is checked via a rubber plug or grommet at the top right side of the case. If you want to drain it you will probably have to remove the right hand rotors and turn the machine on it’s side over a suitable tray. May be as well to drain the engine first . If there’s not much oil in the case, start by adding a small quantity and running the machine just to check that there’s not a gross leakage from around the rotor shaft bearings. Dismantling the case to change the bearings and seals is quite a performance, so if there’s a small leak and the machine is not going to get heavy use it could well be advantageous to add semi fluid grease to the case rather than oil.
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