Gents, I have an 08 Sportster recently purchased and want to start doing some country rides. It has 27,000 km on it. Is there any component likely to be due for replacement that I should be checking? I'm thinking of things like the drive belt, for example. Will be changing oil and filter, spark plugs. I haven't looked at the air-filter but can't see that being a problem.

Drive belts can go 200K

tension check maybe , but  in 22 years I've only adjusted a handful of belts after the first service

Plugs , well yes , oil and filter of course

If it's K&N , cleaning and re-oiling easy enough. If Genuine stock HD filter ( unlikely !) DO NOT replace , get a K&N

Oil the clutch and throttle cables and you're good mate. Brake fluid "bleed" probably a good idea.