So it strikes me that an Earth-mass Mars, in a slightly different orbit would be realistic?
Completely so. Even the orbit would only be different by a very miniscule degree, enough that you could ignore it as a rounding error for AH story-telling purposes.
As to whether it would be warm enough, on average, on the surface to retain liquid water, IDK. I don't know how to calculate that, it depends hugely on the composition of the atmosphere as well as the average distance from the Sun. I do remember that Kim Stanley Robinson had regions on the terraformed OTL Mars in his Mars trilogy where the temperature was above zero, I feel fairly sure he'd have asked someone in the know to get some sensible data but I couldn't guarantee it.
Judging from
this, the average surface temp of Earth absent an atmosphere or any oceans, ice or life would be roughly 274.5 K, or 1.5 °C. The actual average temperature is usually given as 14 °C, or about 287 K, a difference of around 12.5 degrees.
This NASA page reckons that Mars has a current average surface temp of roughly -63 °C / roughly 210 K. Mars does have an atmosphere at present but it's really, really rarified. So it might be reasonable to just apply the same difference of Earth with vs without atmosphere to Mars, implying that just adding an Earth-like atmosphere could raise the average to 222.5 K or -50.5 °C. But the average temperature is related to the planet's insolated surface area too, i.e. if Mars were bigger then the temperature would be higher as a result, although if you found some way to increase the mass without increasing the radius hence surface area then this point would be moot.*
It's also just the average over the whole planet over its whole year. OTL landers have measured ground temps of up to the high 20s °C during local summer. Local temperature can also go higher as you get deeper into the atmosphere, so the bottom of the Valles Marineris will be warmer than, say, Hesperia Planum at the same latitude.
So it still feels reasonable that an Earth-diameter Mars with an Earth-like atmosphere could definitely have a temperate climate near the equator, though I reckon the equivalent of tundra will kick in a much lower latitudes than on Earth, and the polar regions will reach much further down from the poles than ours do.
*Wouldn't be the only issue though. If you kept the radius (0.53 of Earth) the same but increased the mass, density would increase to 6.717 times Earth's, instead of the OTL 0.71, which would give an average surface gravity of 3.56 g. Increase the mass to match Earth's but keep the density the same, then the radius ends up larger than Earth's (c. 1.12) and surface gravity is only about 0.8 g.
