Convert Classic ASP pages to Razor Pages (.Net Core) - SQL Server (Authorize.Net)

Have an existing online entry/payment process written in Classic ASP using SQL Server 2008 R2 database. Accepts payment using MD5 and need to be updated to SHA-512.

Need to update part or all to Razor Pages (.Net Core 2.2) C# using exact same SQL Server database. At the very least, need to update the payment process to use SHA-512. Current app uses Authorize.Net SIM and need to move to Accepted Host (payment processing is all done at Authorize.Net and response is sent back to application).

I am SQL Server developer and wrote original Classic ASP and am "slowly" rewriting but need a "jump start" to get the online payment process up to date quickly. Not complex code (entering a horse show).

Have other small rewrites if this goes well.

Current site - www.showsecretary.com (ENTER ONLINE button from the Calendar page is the current rewrite)

Prefer freelancer with Authorize.Net experience.

Authorize.Net documents:
https://support.authorize.net/s/article/MD5-Hash-End-of-Life-Signature-Key-Replacement?utm_campaign=19Q2+MD5+Hash+EOL+Developer+Final&utm_medium=email&utm_source=Eloqua

https://support.authorize.net/s/article/Do-I-need-to-upgrade-my-transaction-fingerprint-from-HMAC-MD5-to-HMAC-SHA512-and-how

https://developer.authorize.net/api/upgrade_guide/