No luck tracking down a new timing idler shaft.
I did find two on ebay in the US but they were both asking more than a complete engine is worth, so didn't bother bidding on those.
One would probably turn up eventually (or a very cheap parts engine at least) but I want to finish this ASAP so decided to pull the timing gear out of my spare engine.
I was hesitant to start pinching parts off that engine because it was a runner. Only just though, it really needed new valve guides.
To pull the timing cover, the sump needs to come off - and that was a problem because the engine was sitting on a trolley, and I gave away my engine crane when I moved house so lifting it was going to be fun

I took some rough measurements and made a very basic stand to hold the engine by it's mounts, then spent a very tense hour slowly lifting the engine off the trolley with a pair of jacks until it was high enough to fit the stand.
Then quickly dropped the sump and pulled the timing cover (while being careful not to bend the oil slinger like I did on the GTV) and grabbed the timing gear.
The oil pump drive gear didn't want to come off at first, and looks like it might have been spinning on the crankshaft, which would explain thy the engine looks like it's been starved of oil at some point.
The idler shaft inner bearing surface is a bit scored (again my guess is low oil pressure) but it should be ok after a light buff.
The condition of this engine compared to the GTV's is startling. I got this GTV about 16 years ago as an abandoned project that had been left to rot in a yard, but there was a glovebox full of receipts for mechanical work carried out before it's demise - including an engine rebuild.
Since I've had the car it's done about 4 track days, a few laps of my old driveway, and roughly a dozen donuts (mostly in the hands of my daughter) so probably less than 100km all up.
Here's what it looks like inside (without any cleaning) - compared to my spare:

- GTV block
- block1.jpg (53.64 KiB) Viewed 4386 times

- GTV oil pump/timing cover
- block4.jpg (100.35 KiB) Viewed 4386 times
Spare engine:

- spare block
- block2.jpg (119.95 KiB) Viewed 4386 times

- spare
- block3.jpg (133.82 KiB) Viewed 4386 times
And my gaskets turned up today, so I can start re-assembling it all tonight
