Andrew, I'm inclined to guess that the manufacturer didn't intend the mower to be used for slashing, hence the lower belt tension when the deck is fully raised. That doesn't mean it can't do the job, but it may need some modifications here and there. There is a Victa parts manual available, and it is on this site in the manuals section, but the file seems to be corrupt. Hopefully it can be replaced.
I seem to have read that Victa (an Australian company) didn't make ride-on mowers, it sold mowers made by another manufacturer and labelled them Victa. The 5-26 may be an MTD model, but I don't know which one. If you think you have the right manual, try comparing some specific mechanical and electrical features with yours.
If you can find the part numbers you need, check the on-line store on this site to see if the parts are available.
