You will need a "Composite to HDMI converter". Just google for this and you'll find lots of options. The onscreen menu is only available from certain video outputs on the HTS (like the composite or S-Video output).
You might have to look around for a compatible remote. Here's an option:
https://www.remotes.com/krell-p/KRELL-RT305166.htm?gclid=EAIaIQobChMI_YmsgZH34gIVBr3sCh1nPQjBEAQYASABEgLInPD_BwE
Sometimes you can find an original Krell HTS 7.1 remote on ebay. Or maybe this, but very expensive:
https://www.remotes.com/krell-p/KRELL-305166-G.htm?gclid=EAIaIQobChMI_YmsgZH34gIVBr3sCh1nPQjBEAQYAiA...
You might have to look around for a compatible remote. Here's an option:
https://www.remotes.com/krell-p/KRELL-RT305166.htm?gclid=EAIaIQobChMI_YmsgZH34gIVBr3sCh1nPQjBEAQYASABEgLInPD_BwE
Sometimes you can find an original Krell HTS 7.1 remote on ebay. Or maybe this, but very expensive:
https://www.remotes.com/krell-p/KRELL-305166-G.htm?gclid=EAIaIQobChMI_YmsgZH34gIVBr3sCh1nPQjBEAQYAiA...