the problem is whether or not the recess the seal sits in is machined to provide a positive stop to locate the seal?
if it is: knock the seal in with a hammer on a socket until it stops.
or
if there is no stop: measure how far back the seal sits BEFORE YOU REMOVE THE SEAL. of course, you need to do that anyway, just in case.
then
knock the seal in as above, stopping at the correct point (tricky).
or
make an approximation of the correct tool.
whatever....forget the lolly sticks.