Something to remember with Soviet legacy vehicles is that the sights, periscopes and so on tended to be smaller in the armour penetration than earlier versions. This means you can generally cannibalise a periscope off a T-72 and jam it into a T-10 with a simple adaptor plate.
What I'm getting at is that GMs shouldn't assume that the old vehicles have old radios, periscopes and attendant night vision systems, gunnery sights, subsidiary weapons, smoke launchers and all that junk. That all might be pretty modern because it makes more sense to drop that into a hull you're getting ready to refurbish than try and sources parts for old stuff.
Also the older stuff tended to be fairly thick in the armour department, thick enough to take substantial ERA packages.
As to engines, keeping an old clapped-out clunker from the 1950s makes little sense if you have spares from more modern vehicles in store. Not only are they more powerful but they are also lighter. Swapping an engine isn't as difficult as many people might think.
When I put these vehicles into play I like to make whole new vehicles out of them rather than use some old nostalgia vehicle. Not only is it a fun exercise and really they only turn up rarely but also it keeps the players guessing. Especially when the T-43/85 has a radar sight
