Haven't done an exhaustive search, but I don't think there are many, if any, direct replacements. There are similar types, but one parameter or another is different, such as plate dissipation of the pentode or mu of triode is different, (check pin arrangement too) etc. Since parameters are different, one would, of course, need to optimize the design for that particular section. Here are some similar ones and prices at Antique Electronics:

6BM8 $10.10 at Antique Electronics supply
6GV8 3.80
6GW8 8.60
6HF8 4.85
6JT8/6KR8 2.50
6KV8 2.00
6LQ8 3.00

Pentode section may be called Sharp Cutoff Pentode instead of Power Pentode and plate dissipations are less on some listed. Might be worth a try though.

Take care.
Steve