They're not rare Chris !! Having watched for CS's on ebay for a couple of years now they regularly appear but occasionally there are times when they dry up for a while. They also appear in SE mag almost every month, but you have to get the mag on the day it's published and be on the phone immediately. Although a tractor would drive anything you manage to attach to it it would use more fuel and be far noisier than a CS, also , do you know how much a Field Marshall would cost you ??!!
Another engine you could look for is a Petter AV1, runs at higher speed but is a good solid engine. Also, as has been mentioned in another thread, you could look for an FR series Lister, another good, under appreciated engine.