You can run whatever tyres you want, as long as you compensate for that with your gear ratio
I just mentioned that so that in case everything else is original, you're not wondering why it burns clutches/belts like crazy
Sorry, not much info at all..
IIRC it came originally with a 5hp briggs and a series 30 TC, but IDK much which ratio it had originally..
I see your Torqueconverter has been replaced with a std belt pulleys which of course is removing all the benefit of having a TC and rendering your ratio nearly useless I'd assume.
Again, if I'm not mistaking it had 20" tyres on the rear and large diameter but narrow fronts (more like kid-bicycle wheels but sturdier of course) as well as a bicycle-style handle bar instead of a steering wheel.
I've seen a few around the forums and a sibling or two (slightly different Bird chassis)
Sorry, apart from that not much I can tell you about it.
'sid
[
EDIT]
this is about as original as it gets (uhm bad shape but still

):
