Are you using friction shifting (no clicks)? Is the crankset a double with standard size rings, 52/42 was pretty common in '85? Do you want brand new or used?
Based on the 30T cassette, a mountain bike RD would be good option. If you use friction and buy used, most long cage models should work fine. Many older short cage RD won't comfortably do a 30T cogs or have a the total capacity for a 29T wrap. Older MTB RD can be found at good prices. If you currently use a 9 speed chain or plan to soon, a 9 speed RD has slightly narrower pulleys, but this not too critical.