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Solaris x86 wasn't a supported installation target for R70.50.
That means you need to use SPARC Solaris on actual Sun hardware or something capable of emulating it.

Assuming you can do that, it should be possible to migrate the configuration to a current version through a series of upgrades.
I believe the last version supported on Solaris was R75.40, which you should be able to upgrade directly to from R70.50 according to the R75.40 Release Notes.
Once you get to R75.40, you can use the R77.30 migration tools and import the configuration into a new Gaia install running R77.30: https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solut...
From there, you can upgrade to R80.40, then R81.10 (either in-place or using the migration tools).

However, it might be easier to use the following tool to extract some of the configuration from your backup that might be useful: https://community.checkpoint.com/t5/Management/Exporting-Importing-R77-x-and-Earlier-Configuration-o... 
You just need the objects_5_0.C and rulebases_5_0.fws file from the backup you have.
You can then use this tool to "import" the information into an R77.30 installation and then use the migration tools to copy the configuration to R80.40 then R81.10.

Hopefully that helps.