Unfortunately they are all internal, there are no external versions. For the cost of the slave cylinder it is worth changing if the engine has to come out.
There are a small number of M3W's with the external clutch slave cylinder. See page 20 of the Parts book for details. It looks like MX5 parts. Agree that the internal slave cylinder is worth replacing when the engine is out, they are fairly cheap, unless you buy a genuine Volvo or Morgan part.
