You could try rubber rejuvinator or just get the dimensions and buy a new tire. Parts are getting hard to come by!
From
http://www.russellind.com/prbline/
VC-6847 ...........REEL . . . . . . . . . . . . . . . . . .1425-64
...................LOADING . . . . . . . . . . . . . .1407-464
...................SENSOR . . . . . . . . . . . . . . .1425-22
...................IDLER ASSY. . . . . . . . . . . . .1408-144
...................IDLER TIRE . . . . . . . . . . . .1428-91
Specs here..
http://www.russellind.com/EVG/idler.htm