Hello John:  I can tell you what we use in place of #320 plastic glue:
"Aleene's Tacky Glue" in the brown bottle.  Here in the States, we buy
it at the local Wal-Mart, in the crafts department.

This glue is very close to plastic glue.  It can be used safely on
players with needs for plastic glue, pouches and valve covers.  If this
glue is thinned 50/50 with water, it makes a very good wood sealer as
well.

You might try the better-equipped craft shops for the brand name.  We
have also have used what is called "Modge Podge"; it is also plastic,
but it dries slower.  We have recovered the small two-tier pneumatic
actions with this glue and it's holding up great.  If you do use
plastic glue with pneumatics, make certain that the hinges are replaced
and double-hinged as well.

Hope this helps.

Chris Taylor
Tempola Player Piano Rolls