I've seen a few successful "upgrades" on some Estes Saturn V's, but only like 2
Here is one of them I found at the Smokey Mountain Amateur Rocketry Team web site.
https://smartrocketry.com/INTRO/Member_Bio_s/Kirk_S/KS_SatV/ks_satv.html
The font on thes page was small, hurt my eyes so I'm pasting the article below, but there are some pictures at the above link as well. Good Luck! The guy that wrote this article did a really good job beefing his up. Hope the info helps.
<b>Motor mount:</b> I replaced the kit motor tube with a 29mm tube of the same length. The centering rings were replaced with ones made of 1/8" plywood. To make a motor retainer I put 8-32 T nuts in the bottom and use 8-32 Allen head screws to secure a conduit reducer over the motor. I drilled some holes in the forward centering ring to run a length of picture hanger wire through for a shock cord mount. I made a loop with the wire at both ends and secured the loops by crimping and soldering them.
<b>Structural reinforcements:</b> The vacu-formed fins and shrouds would never survive so I strengthened them by filling them with a mixture of epoxy and micro balloons. I forget what they recommend using to secure the decorative wraps to the outside of the rocket, but I used contact cement like they recommend for their Mercury-Atlas kit. All the centering rings for the body tubes were replaced with ones I cut from .040 inch sheet styrene. The paper transition section shrouds were replaced with one cut from .020 inch sheet styrene. I used 30 min. epoxy to glue everything together except for the contact cement already mentioned and plastic model cement on the shroud seams. I was concerned with how sturdy the airframe was forward of the motor mount so I put a layer of .020 inch sheet styrene inside the tube with contact cement being sure not to glue anything in the first 1 1/2 inch of the airframe so the upper section would still fit inside.
<b>Recovery system:</b> The shock cord was replaced with two pieces of 1 inch wide 3 foot long elastic I bought from Wal-Mart. one end of the first was tied to the shock cord mount and the other end to a quick link. One end of the second cord was attached to the quick link and the other end was tied to a 48" PML chute. I used the metal hook supplied with the kit as the attachment point at the service module area. The shroud line that is feed through centering ring at aft end of the upper section was replaced with a much heavier piece of nylon string. The shroud line/hook assembles that attaches the upper section to the shock cord were replaced with 24 inch wire fishing leaders. The loop ends of the leaders are attached to the quick link between the two shock cord sections, the hook ends are attached to the two loops in the upper section. I also tied a piece of string between the escape tower and the loop on the service module to secure it to the rocket in case it broke off. (has come in handy once already)
<b>Stability:</b> I entered all the rocket measurements in VCP and calculated the center of pressure to be 13 3/4 inches from the aft end of the rocket. I added enough weight to the upper section to put the center of gravity 4 inches forward of the CP. I did not add the weight all the way forward in the upper section but placed it in the very center of the upper section. This required more weight but helped the upper section balance better in the horizontal recovery attitude. The weight was secured inside the "third stage" body tube by gluing a .040 inch styrene bulkhead inside it at the proper balance point, after this dried a mixture of epoxy and BB's was put in on top of it and a second styrene bulkhead glued in on top of that. The total weight of the rocket came out to 4 pounds fully loaded with an H97 motor.
Oh, well................
