Never seen ANY AR with oil-filled caps. Every one I have been into has used either separate electrolytic caps OR computer grade electrolytics sealed in a wax block. Most of the ones I have encountered have drifted and deteriorated over the intervening 30 or 40 years. My understanding is that the dielectric paste separating the plates of the capacitor degrades with time.
As for the seal? Oh just about sealant will do - caulk - if thick enough - foam gasket, rubber gasket. The seal doesn't have to be so tight that it will hold air for more than a few milliseconds - as the woofer is moving constantly. I have used RTV (silicon sealer) with good success. The only problem with RTV is that it tends to be too sticky - and after using it removal of the driver will cause the old OSB board that the cabinets are made to come apart. Bad.
Good luck - the AR2-ax with new L-pads and a reworked crossover is sonically a blissful speaker.