1. You can make the client stay with the old classic 32-bit asp, but distributed on several web servers, each with 4 GB of RAM to help improve overall performance until they can migrate the entire site to ASP.NET or PHP. Using both is not the most brilliant and discouraged idea. If you want to use an enterprise-class technology stack with all the built-in ringers, you should move on to building ASP.NET sites with C #.
2. Stay with the current route and get a bigger headache.
3. Involve a team of people to talk faster and put aside the dead technology so that the site can be standardized in ASP.NET or PHP. If they have trouble finding people, PHP would make it easier. If you need the features of the company that are integrated along with the security features, then you should continue with C # and ASP.NET.
4. Hire a consultant or study to do the work for them if they can not do it with the current staff, the contractors or hire the right talent in the company. In general, it seems that some bad management decisions were made and now they are dealing with the technical debt of making bad decisions that season engineers and technical management would not have made. Either way, the technological debt is there and the problem can be solved quickly or slowly, depending on the priority of moving the stack to something 100% modern in order to maximize the benefits of doing so (lower CPU usage, faster site , easier scalability), better built-in security, etc.).
5. Technically, you can put the site behind cloudflare, squid or a load balancer and divide it into multiple virtual servers while taking the slow route and update the pages to ASP.NET while caching and redirecting the older sites to the pages newest with 301 redirects, add a site map to your site and send the site's maps to the major search engines.