You could drain the oil into a clean container and reuse or if you're brave, lean it against something. (I did this once with a CB175 and it fell on me, so I'm a bit cautious).
You can specify an SKF seal which, in theory, should be guaranteed quality for £9.74 + VAT. (double lip)
https://simplybearings.co.uk/shop/Se...ex.html?page=1 (you'll have to plug your numbers in).
If you go direct to Fowlers page, the seal is listed at £43.04.
https://www.fowlersparts.co.uk/parts...7/transmission
If you google the Suzuki part number
09283-42017-000, the size is shown as a 42x62x7.
I'd buy an SKF seal.