My previous strong recommendation is unchanged -- replace this very old and overdue timing belt, belt tensioner and roller as soon as possible.
The belt may look OK superficially on the surface but without X-ray vision it is impossible to know what it looks like beneath the surface.
Leaving the ancient belt in place carries the risk of further deterioration (when will you next look at it?) and more importantly carries the huge risk of very expensive engine damage with lots of cost and lots of inconvenience – and taking this risk achieves what exactly?
The 1HD-FTE is an 'intererence engine' meaning that if valve timing is lost (such as when a timing belt fails or slips due to failed tensioner, or is cooked riding over a jammed roller) the pistons will crash into open valves and the damage would be immense -- broken valves, bent crankshaft, damaged pistons, scoring of cylinder walls, possible bent connecting rods .....
Suggest use the internet to check on prices for a replacement 1HD-FTE turbodiesel engine – very expensive even in used condition -- if a good one can be found and assuming local expertise is available install it.
Installing a replacement timing belt kit is very cheap insurance against these risks.
As you will have seen in the videos back at my Post #5 in this thread, the task requires considerable care but it is a reasonable do-it-yourself task. And it is very different and much, much easier than a timing belt replacement on 2UZ-FE gasoline V8 engine.
If it were my vehicle, I would enlarge the project and replace the ancient water pump as well, as shown in the second video. This is also cheap insurance.
In the last pic in Post #5, the pulley which is attached to the crankshaft can be seen. The correct size socket, a short extension piece to reach into the pulley, and a good long handle for leverage will be enough to turn the engine, even with the very high compression in the 1HD-FTE turbodiesel.
If only replacing the timing belt kit, it will be easier to turn the engine from the below.
If also replacing the water-pump, then the fan and other things must be removed. Then the engine then can be turned by reaching from the top or the bottom.
For the IHD-FTE turbodiesel engine, the Factory Service Manual mentions FIPG (means Formed In Place Gasket) packing material as a seal for the timing belt cover cover rather than an actual pre-formed gasket and refers to Toyota Part No. 8826-00080 “or equivalent”. Suggest make inquiries at a Toyota dealer or an automotive store. Maybe a tube of suitable sealant which can withstand high temperatures near an engine is all that is required to keep out moisture and dirt. There is no pressure to be resisted. This eBay site gives the idea:
Toyota FIPG Black Silicon Sealant - Engine for sale online | eBay - https://www.ebay.com.au/p/655286175 but you will need something more local to you. Suggest take the pictures with you to obtain advice.