What is the proper way to stat a gas 3 wheel Harley golf cart?

You question is very vague as to what type of push cart your needing wheels for.

Taking a guess your describing a 3-wheel GOLF push cart, this site sells replacement parts. (If not a golf cart, please post a new question describing the make and model of cart).

https://news.sunmountain.com/golf-push-pull-cart-parts/

Which Model will determine the year...
year serial # model 1984 - 1986 3B G3
1982-83 3B D3

E-Z-GO Golf Cart Brake Shoes - Genuine E-Z-GO Parts and Accessories

https://shop.ezgo.com > Shop > Parts > Brake Parts > Brake ShoesOEM Golf Cart Brake Shoe 4084 ' Single Shoe. Fits 1996-current E-Z-GO Golf Car and Utility Gas & Electric Vehicles. Item #: 648037. $24.70/ea. Spring- Shoe Return- LH (Blue). Used on 1996-Current E-Z-GO Gas & Electric 4/6 Passenger Shuttle, ST Express, 875, 881, 950, and Cushman Titan Vehicles; in addition to ...

Check the electric shift switch relay with a good VOM (meter) if you have the EL model - or the mechanical linkage if you have the usual gas version. Use a large screwdriver to pry the reverse lever manually back and forth. Be sure to block up the vehicle before you mess with the transmission while running the engine. Safety first.

Hopefully, the repair simply involves a case of wear and tear reducing the necessary throw distance of the shift lever.

If not, then the transmission or drive train needs mechanical, professional, inspection repair or replacement.

Need to find the speed governor and disable it

I found the manual for you're here in PDF format
http://www.buggiesgonewild.com/gas-harley-davidson/41795-free-harley-golf-cart-manual.html

You can shift between 2Hi and 4Hi when the truck is moving. Just let off the gas and shift it. But in order to shift to Neutral or 4Lo you need to stop the truck and shift the transmission into neutral then shift the transfer case.