Once you have installed your mechanical seal (or if you’re simply replacing the camshaft oil seal) you can install this seal (part number 91202-283-013, available from eBay US, eBay AU, eBay UK, eBay CA and Amazon). Looking for the front oil seal instead? See this article.
If you are going non-genuine this is a 18x28x6mm oil seal. The old part number for it is 91202-283-000.

Rear side (mechanical seal size):

Oil-facing side (front of engine, away from the mechanical seal):

It should be easily able to be pushed in – you can use a socket to make sure you’re driving it in straight, but it shouldn’t require a great deal more than light taps to push in. Only push it in so that it’s flush with the case, as you don’t want to obstruct the weep hole:

Here’s how far it should go in; you can still see the weep hole, circled in red here.
Once it has been installed lubricate the inside (where the camshaft will sit) with a tiny bit of fresh engine oil – this will help reduce the chance of damage to the seal on installation/first start.

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