Hi Pete,
Your symptoms do sound like an air leak. provided the manifold screws are tight then the crank seals are the most likely culprit. I would guess it was the generator side and its easiest to start with that as you only need the minimum of tools.
Job can be done in frame as follows.
1.Remove dynamo stator – 2 screws and note where wires go. Extract rotor, you need a long threaded 8mm bolt to pull it off the crank.
2. Inspect seal for obvious signs of leakage or damage but since its sucking inwards there may not be much.
3. Remove the circlip which retains the seal. There are two types, early ones are proper flat circlips, later are just wire.
4. remove seal by carefully drilling two small holes near the edge of the seal 180degrees apart. Do this carefully as you don’t want to damage the plate behind the seal.
5. Screw in two self tappers – gently again and they may well force the seal out. If not, once they are firm use pliers to pull the seal out.
6 Fit new seal – inner side to engine.
7 fit circlip and dynamo bits – job done.
Probably best to get the seals from TheMZshop but if you are confident in ordering such parts they can be bought for less from a bearing stockist such as Simply bearings. Spares book will give you the sizes 72 x 30 x 7 from memory.
There is a better seal for the drive side with double lips but these should not be used on the generator side.
If that does not do the trick then you may have to tackle the drive side which does need special tools. However its less likely to go as it runs in oil. Come back for more help if necessary.
Peter