A good source with pictures of S/N locations is http://people.clemson.edu/~wsmth/SNLocat/ACSN.htm
D17 Tractor S/N should be on flange where transmission mounts to engine on drivers left side if I recall. Engine S/N should be on block in flat space behind carb. Motor s/n should start 17- and have a letter suffix. Both S/N will be steel stamped into the metal - not raised - the raised numbers relate to the casting.
WD and WD45 S/N should be on rear of tractor axle brake housing area - again on drivers left and both should start WD followed by up to six digits. Some early WD had S/N on top at edge of (possibly under) left brake cover. WD were under 146606 and WD45 started at 146607. Motor S/N same as D17 except it should start with W or WD for the 201 ci WD motor or 45- for the 226 ci WD45 motor.