About the moulded weight, if you manage to get your hands on the old Accurate kit (NOT the reboxing), they are included! I managed to track down their sharkmouth 25B and lucked out on an american online shop without getting ripped off by customs.
The glass house makes it tricky, the best place to put them - you don't have much choice! - is the bombardier tunnel (heavy fishing weights) and MAYBE add smaller weights under the greenhouse's floor (but I didn't try this one).
I saw some modellers putting additional weight in the front part of the engines, totally hidden by the kit's parts but in front enough to move the gravity center forward. Once again, I didn't try.