My first Ibby was an EX370 bought new in '93, so I can give you the full scoop. I still have it, believe it or not.
The EX 3XX guitars all came with the LO-TRS trem, and to the best of my knowledge were never made in Japan.
My EX now has a LO-TRS II from a more recent RG400-something with a brass sustain block, an OFR Arm Holder, and Edge studs which have all made it a lot better than it was.
An OFR will fit right in, with the normal TRS to OFR gap on the bass side of the tremolo unit. I hate having the gap there, otherwise I'd just put in an OFR and be done with it.
These guitars were pretty decent for the money back when and can be very capable instruments with the appropriate upgrades in hardware. Basswood (non-ply) bodies, decent necks, etc. They compare more or less to 3xx RG's now, as low-end RG's didn't exist when the EX-3xx guitars were out. It doesn't hurt that you can buy them for milk money these days.
Hope that helps!