Dear one,
You should read the Arya Sanghata Sutra twice. Start reading the sutra right now, so you can finish it before the surgery.

You can also look at a painting of the Buddha, and visualize beams are emitted from the Buddha, who is looking at you with loving kindness and compassion. Beams are emitted from the Buddha and enter you and all sentient beings, purifying you and all beings, and liberating every sentient being from all the sufferings, negative karmas and defilements. Then every sentient being, as well as yourself, becomes in the nature of light, becomes a buddha and receives all the qualities of the Buddha. Then at the end, Buddha absorbs into you and all sentient beings.

Recite Guru Shakyamuni Holy Name and Mantra:

LA MA TÖN PA CHOM DÄN DÄ / DE ZHIN SHEG PA DRA CHOM PA YANG DAG PAR DZOG PÄI SANG GYÄ / PÄL GYÄL WA SHA KYA THUB PA LA CHHAG TSHÄL LO / CHHÖ DO KYAB SU CHHI O / JIN GYI LAB TU SÖL

TADYATHA OM MUNE MUNE MAHA MUNEYE SVAHA

To Guru, Teacher, Bhagavān, Tathāgata, Arhat, Perfectly Complete Buddha, Glorious Conqueror Shakyamuni, I prostrate, make offerings, and go for refuge. Please grant me your blessings.

Please recite this every day, not just for the healing, but also for enlightenment for sentient beings. Everything should be done with that motivation. Recite at least two malas every day if you can.

You also need Lu Zong puja, to be done twice at Kopan.

Please recite the Verses for the Eight Noble Auspicious Ones (Tashi Gyepa) every day if you can.

You also need to wear the blessed strings that you would have received during long-life pujas, etc. Put them on and wear them during the surgery.