Tax Receipt (CDN TAX RECEIPTS)
Since it's a ts() wrapped string, you should be able to use Administer - Customize Data - Word Replacements to replace it. See https://github.com/jake-mw/CDNTaxReceipts/blob/1.5.1/cdntaxreceipts.functions.inc#L409. I haven't tested that.

To fully customize it you could implement hook_cdntaxreceipts_writeReceipt. See https://github.com/jake-mw/CDNTaxReceipts/blob/1.5.1/cdntaxreceipts.functions.inc#L301