what is wrong with the stock OPT? blown or you just want different tone? better dependability?
have you checked out the Orange mod kit by Mercury?
http://www.mercurymagnetics.com/pages/mainframe.htm#catalog/kits/TinyTerror/Orange-TT.htm
yeah, kind of pricey for an 18 watt amp and it sounds like you want to score a more reasonable priced OPT as you have already shelled out for tubes and such, maybe borrow the choke idea,
that looks like a pair of EL84's with a dual master volume which always has been a favorite,
cool dual gain pot also, that is a standard phase inverter drawn in a weird (reads different) way, a few values tweaked but pretty basic.
those tubes want about 7.5K to 8K but 6.5K would work fine. replace the 470 ohm screen resistors with 1 K 2 watt to compensate for the increased load from the 6.5K ac resistance (impedance)
you might try the Hammond 8K output, Antique Electronic p/n P-T1750E. $36.28
https://www.tubesandmore.com/products/transformer-hammond-output-replacement-fender-15w-8k
we can wind you a vintage Fender 8K as used in the 1959 Tweed DeLuxe but that is $50 and has a compressed sound without the brittle hi end, but the hi end on the 6BQ5 is awesome, so you might want something which will bring that out,
we like a 100 EI core with four sections of 645 turns = 2580T primary into a three section 119 turn secondary which sports a center section of 63 turns wound bi-fi for a 7.5K to 16 ohm job as used in the Fisher amps of yesteryear,
it is hard to tell what the 6.5K transformer will sound like just from the Z number. Best way to find out is to try it or search online blogs on Tiny Terror mods, which is probably where you got the idea,. Lower Z would predict less bass due to less inductance, hi end is a crap shoot as it will be determined by the coil, number of turns and wire size, interleaving, size of the core, etc.
have you heard the VOX Night train? that is the favorite glove box amp over here,
http://www.prowessamplifiers.com/schematics/Vox/Night_Train_Updated.html
tiny terror>