If a tank is just a tank, you can use whatever will hold oil with out leaking and can be made to have the appropriate fittings. Things are never this simple though and packaging will no doubt be a problem. I'm not familiar with the BC900 application or the hydraulic system it employs. Some systems use the tank as an integral part of the system and it can be pressurized. Other systems just use the tank to keep that extra reservoir of oil to pull from. If it is the former, you will probably want new or used OEM. If it is the latter then aftermarket would probably be fine.
Ive used Northern hydraulic tanks in several log splitter builds where the tank is only a holding and retrieval vessel for the oil. Hydraulic hoses can be extended if need be accommodate slightly different fitting sizes and or fitting/boss locations.