How do i change the key barrel on a pride pursuit please?

SOURCE: pride pursuit has little power in reverse

I am not an expert on such things but I have had a little experience of a mobility scooter with a similar symptom or fault.

Speed control is by a switch-mode power supply and reverse is by circuitry and relays (or SCR's) that change the polarity of the supply.
The fault on Sam's just-out-of-warranty scooter was caused by an electronic fault in the polarity change circuitry, which was just a small part of the large single circuit board. I approached the manufacturer who told me no repairs were possible and a complete new board would have to be purchased at a huge cost.

I argued the point on behalf of my customer and finally a new board was sent free of charge which put matters right.

I fully expect your problem is likely to have a similar cause, though you might be fortunate enough to be able to obtain circuit diagrams for local repairs, a manufacturer repair or a refurbished board at a reasonable cost.

A possible cause of such faults might be the weakening effect of electricity surges in the supply during battery charging. It would be wise to only charge the batteries when a surge protector is used in the mains supply, such as is recommended for all mains powered electronic devices including computers.

I hope this helps.