Magic Tricks

The back design and the width of the cards don't matter.

Another thing you may want to consider also is that Level One is made for a 52 card deck, a tarot deck has 56, not counting the 22 Major Arcana. So a full tarot deck has 78 cards.
If you look for the Gypsy Witch Fortune Telling Cards, they are of the same size as a deck of poker cards or very close in size which Level One should work.
Penguin Magic does not carry them so a bit of googling is in your future if you need to find them.

The gimmick only works with standard size playing cards.

"Josh, I have now found a tarot deck that is exactly the same size as a bicycle deck (the Tarot of Trees). The white colour of the cards is the same but the back design is different from the standard Bicycle design, so will that be a problem? And if so, can I adapt the gimmick? Many thanks for your advice, much appreciated"

If that's the case, I could see it working just fine.

Josh, I have now found a tarot deck that is exactly the same size as a bicycle deck (the Tarot of Trees). The white colour of the cards is the same but the back design is different from the standard Bicycle design, so will that be a problem? And if so, can I adapt the gimmick? Many thanks for your advice, much appreciated.

The Level One gimmick is made to the size and color or a bicycle deck. If the deck you want to use differs at all it will probably not be a great fit.

I've been looking at the Crowley Thoth Swiss Tarot deck but the measurements as given on different sites, although similar in size to a bicycle deck, are listed as different from each other on each site. Maybe this is a lack of precision with the listings. But if the gimmick needs to fit a bicycle deck exactly, I'm reluctant to buy without some more re-assurance.