Is there a 32-bit SDK available for Xprotect 2019 R1?

Yes.

https://developer.milestonesys.com/s/article/About-Milestone-software-development-Kit-SDK-download-link-For-MSP

https://developer.milestonesys.com/s/article/Processor-architecture-compatibility-for-the-Milestone-Integration-Platform-MIP-SDK

Thanks