Now THERES a thought... We may be the last generation able to 'restore' our old dirtbikes (or roadbikes). Imagine in 20 years, first finding a YZLX450F that isn't blown up. Then, imagine trying to find the bits to do a bit of a basic tidy up.
I reckon that this scenario will prove to be wide of the mark.
Imagine standing around a new 120Y in 1974, or a DT1 in 1969 - bet you'd quickly decide that it would be impossible to restore/maintain one in 2010 - and the arguments would be very similar to the ones being presented here.
Today's 'specialised technology' is tomorow's mainstream.
Stuff like EFI is a great example - I own one of the first widely available EFI cars (1972 Volvo 164E) and despite the horror stories that were still alive and kicking into the 1990s, the EFI is actually bloody reliable, and new & used parts are still readily available.
The absolute worst case scenario is to buy a cheap aftermarket ECU, modern injectors, etc and fit that. Its not hard, it would increase performance & fuel economy, would not appear out of place, and (with some careful shopping) could easily be done for under $1000 all up (priced a rebuilt SU lately?).
Similarly, look at the number of options to replaced toasted old bike CDIs.
What was once 'exotic' technology, quickly becomes well understood, supported and within reach of Joe Average.
And that's even before you explore the issues Lozza has raised, where fabrication is now easier than ever.
Remember that 15 years ago, a MIG welder was considered exotic.
Similar story for TIG welders, CNC lathes/mills, CD burners, etc etc etc.