No problems. It's not 'too big'.
Think of it as the difference between putting Supermarket own brand batteries in a torch, compared with the ones that run the Bunny
The original battery spec, was, according to the manual 43 Ah, or for some reason 1098 engines had only 40Ah..
Ah is the amperage (power) the battery will provide for 1 hour. A 40/43 Ah battery will have a CCA of around 320-380 amp.
Basil has a standard Dynamo system and a 60Ah battery.
I've had him 3 years and he was already fitted with a type 005R, which is 60Ah & CCA 510amps.
(Ideally, Basil should have a 005L, to put the connections the other way around, as the posts are at the front.)
CCA is an important number as it's the Cold Cranking Amps. Different manufacturers of 60AH batteries will have different CCAs.
CCA is a measurement of the current a fully charged battery can deliver for 30 seconds and maintain a voltage of 7.2 volts (12 volt battery) at a temperature of -18°C.
So you can get batteries that are the same Ah, but their ability to turn an engine over on a cold morning will depend on their CCA.
Think of the 60Ah as being an indicator of how long the battery will give you a bit of extra electrical power on a dark winter night when the dynamo is running at max ability.
While the CCA is the ability of the battery to start it on the cold morning.
If the one you have acquired is serviceable, it'll be fine.