I installed a replacement tilt-pack in our bathroom (1920's ) house and it worked great. A bit tricky to do the first one but I imagine the next ones would be easier afet I whew what I was doing.
I am now looking into the Jeld-Wen ZapPack product, to replace other windows. It is pretty much the same thing as the Marvins but I think I can get them for less at Home Depot.