The reason the belt is loose would be my main concern. First I would make sure replacement belts are really same width as original new ones; less width makes them sit lower in pulley which will make them looser, make sure routing is correct, deck is in good condition (take it off and flip it over) then I'd make sure the engine pulley is properly located on the crank after the replacement or if it's not a direct replacement to original engine maybe the replacement engine's crank is lower/higher or just not in same physical location because of mounting issues; if the physical location can't be adjusted then a shorter belt would be an option in that case.

 

Thanks scocope-- deck is in great condition--- replacement engine is a 17 h.p.(drop in replacement) while orginal engine was 15 h. p (10 yrs old)--
Belts were 1/2" width and appear to fit pulleys o.k.---- but they do look loose when disengaged.. Would you recommend a 1 or 2 inch shorter belt???
THANKS again!!

 

Since we are talking mowers (not autos) what the heck; try the 1 inch first. If it appears to be loose DO NOT run it! You should be able to return it if unused and be able to swap it for a shorter one. Be aware that a excessively tight belt might not disengage fully and can cause excessive bearing wear.
I still am concerned why the belt is loose and given that it occurred after the engine replacement, the engine pulley position is the likely cause; "drop in" does not mean everything is in original place, I've seen where the replacement engine pulley had to have a spacer and it was supplied with the kit. However the spacer was incorrect because of JD model changes from year to year that the engine supplier did not know of and a new one had to be machined after measurements were made.
Try to reposition the pulley on the crank shaft to see if that alleviates the issue (and have a spacer made if needed). However if a shorter (a tighter belt) still breaks AND you are sure the deck is OK and the pulley is OK, figure out where the rub is; paint the underneath with cheap black spray paint if you need to be able to see it, find it and grind the offending item away but this is not really a fix but a jury rig; there's still something wrong you are missing.