Great Compassion Mantra in Siddham:
http://www.siddham.org/yuan_english/mantra/emantra_84s_s.pdf

Aksharamukha: Script Converter:
https://aksharamukha.appspot.com/input/Siddham

Proposal to Encode the Siddham Script in ISO/IEC 10646 by Anshuman Pandey:
https://escholarship.org/uc/item/1zx6t75q

The Siddham script (Chi. 悉曇文字 xītán wénzì; Jap. 悉曇 shittan; Kor. 실담 siltam) is a Brahmi-based script historically used for writing Sanskrit in China, Japan, and Korea. It is also known in Japan as bonji (梵字; Chi. fànzì; “Brahmā letters” < Sans. ब्रह्मा brahmā). At present, Siddham is used predominantly in Japan, where it is associated with the Shingon (真言) and Tendai (天台) schools of Japanese esoteric Buddhism (密教 mikkyō). Siddham was used for writing Buddhist manuscripts, but it is now mainly used for ceremonial and ritualistic purposes, such as the copying of sūtra-s and the writing of mantras and bījāksaras “seed syllables” associated with esoteric Buddhist practices. It was also used for inscriptions on statuary, amulets, and other artefacts.

Learn Siddham script at omniglot.com:
https://www.omniglot.com/writing/siddham.htm

siddham consonants letters:

siddham vowels letters:

learn and write anicent siddham alphabet letter:

HOW TO WRITE SIDDHAM ALPHABET CONSONANTS LETTER:

NEAT AND CLEAN|SIDDHAM ALPHABET LETTER:

如來跋折陁羅尼 (in Siddham):

虛空藏菩薩遍照陁羅尼 (in Siddham):

清淨陁羅尼 (in Siddham):

And more of the above (including, for example, Sukhavativyuha Dharani, Bhaisajya Guru Dharani, Cundi Dharani, the Heart Sutra, etc):
https://www.youtube.com/channel/UCfF7fm2GGhHQzrU0gQ5B81g/videos

The Heart Sutra in Siddham: