In the case of normal maintenance I've made a few mods to my 505 lately.
The viscous fan (finally) packed it in so I got some EF Falcon fans and shroud off of ebay for $50 and have fitted them. The process involved removing the radiator, modifying the falcon shroud a bit with the hacksaw, chopping the viscous fan pulley off and then fitting radiator and shrouding back in. It's pretty tight clearance-wise but the radiator shroud actually fitted really well in the end. One of the falcon fans is a two-speed job which I wired to the existing thermofan relays and the other fan I've wired to switch manually in the cock-pit. I'm in the process of installing a Tridon TCS030 coolant sensor and jaycar variable voltage relay to control the second fan though. The airflow when both fans are running has to be seen to be believed and is a massive improvement over stock. Slow running and traffic light grand-prixing with air-con on on a 40 degree day will simply not budge the coolant needle now.
The second mod was to get rid of the POS non-oem exhaust that rubs on the chassis in numerous spots and leaks like a sieve. I went for a 2.5" from the turbo back with a single straight through centre box. The new exhaust is actually quieter than the old system and requires significantly less throttle at 60km/h which is all the testing I've been able to do for now. Before the forum crash there was a thread where someone had fitted 3" to their 505 turbo diesel but 2.5" was the biggest the shop were able to do whilst still going over the rear subframe.
More info and pics to follow.