I am not aware of anything that can exactly replace fish hydrolysate. Humic acid could work as a substitute, but there is some conflicting information about whether or not it really is a good fungal food source. For 50 gallons the Compost Tea Brewing Manual recommends 500 mL per 50 gallons (take a look at the link I posted for the full fungal tea recipe).
Yucca can also be a good fungal food source, but I have not personally used
yucca much so I can't really say for sure. If you don't have any, you could probably just skip it - molasses, compost and seaweed are the 'essential' tea ingredients in my opinion.
Fish hydrolysate is usually not that expensive, you can even order it online from Neptune Organics. Biologic Systems are sells organic fish hydrolysate and it's cheaper than Neptunes Harvest, at least in my area. Not sure what area you are in, but try checking around at Ag supply stores, especially places that retail to vineyards and farms, they are more likely to have affordable, quality ingredients than a hydro store.