quince or canned pears instead of poached

I would use "canned" pears, preferably ones in jars, not cans, as those will be firmer. (It requires a much higher heat to process in metal than glass, so tinned foods tend to be mushier as a result.) If you don't like pears, then poaching apples or quince until somewhat tender would be a perfectly fine substitute in that almond pear tart. Quince tend to be a lot of work, I've found, so if you could get canned quince, that may be easier. I've never bought or eaten any, so I don't know if they would be firm enough. Quince generally are a firm fruit, however, so I'm guessing that would work. Let us know what you use, please, and how it turns out! Thank you. ;o)

Alan -
If you're using canned or bottled products, apples and quince are both delicious and often recommended as replacements for pears. And quince is delicious, if a fruit new to you.
If you can't find packaged whole or half apples or quince, consider applesauce or quince jam. Both would be a serious modification of the Lebowitz recipe - reduce quantities vis-a-vis original fresh, and also some of the sugar. Use them more as a spread than the traditional fanned cut fruit.
If you want to use fresh produce, better to wait until those two fruits are in season where you are.