[
Does anyone know where I might find a buttplate?]
http://www.gun-parts.com/buttplates/ - Bob's Gun Shop, $25, call AM only.
[
Was the original plastic?] - Yes
[
Was it embossed with the Marlin logo?] - Yes
The issue white line spacer can be duplicated via cutting the side out of a large (empty/rinsed) white plastic Clorox bleach bottle, popping two holes appropriately spaced for the BP screws, attaching it under the BP and carefully paring off the excess with a sharp whittling/pen knife blade.
.