Hi KCJim,
Thanks for PM’ing me on this one, I don’t get around to reading all the posts on Ratrod so I am bound to miss out on occasion when my name comes up.
The question is; “is this frame rare and should it be preserved as-is due to its rarity?”
This question has me thinking about (and writing) a longer piece on the subject but a clear and plain answer will probably serve this thread best.
The bike is a girl’s Monark 4-Bar, a companion to the Boy’s 5-Bar model. Monark produced a variety of multi-tube steel frames from roughly late 1937 through about 1941. None of these frames are Common but only some of them are truly “Rare”, with a capital R.
A good way to judge its rarity overall is to note that this frame varient probably turns up on eBay about three or four times a year. On that basis, I would rate the frame as uncommon but short of what I would consider rare.
The springer fork is the earliest Springer produced by Monark and predates the common twin-spring fork that has been reproduced. The switch from the early fork to the later fork occurred at the end of 1939 at which time the girl’s 4-bar frame also seems to have been redesigned with the four down tubes being placed further apart vertically probably in an attempt to stiffen the frame.
So, the frame is uncommon but as a girl’s frame is not sought after to the degree that is has become particularly valuable in the Hobby.
The fork itself is sought after, the redesign to the twin-spring plunger design with fully articulated links was likely due to the fact that the early design is fascinating but shy of being a well designed piece of engineering. Monarks as a whole seem to be failure prone, and the steel Monarks were probably discarded in large numbers when the broke. They don’t just continue to exist like the aluminum frames for lack of rusting capabilities. Many of the early steel frames that did survive have replacement forks due to the fragility of the early fork and owners of those frames are usually searching for good correct replacements. Supply and demand mean the fork is probably worth at least $150+ to someone.
In today’s market I believe what you have is worth about $300. I’ve seen similar bikes advertised for twice that but that gets into no-sale territory. If it were a boy’s version in similar condition then I would expect it to be a worth about $450 to $550 with asking prices probably higher yet.
Everyone values the dollar based on a number of factors in their life so I can’t say if the dollar value of the bicycle will be a significant reason not to modify the bike. The other factor to consider is the Historical Significance of the bike to determine whether it is a candidate for modification or better left as is. This too ultimately is a determination the owner must make.
While the bike is not wildly valuable in dollars and not rare to the degree that other equal or better examples cannot be found, I think that it does have a good degree of historical significance as it sits. Considering the overall decent condition of the bike, including the fact that it has not been repainted and that it is an uncommon model, I would not personally do anything to the bike in my ownership that was an irreversible modification on the grounds of my interpretation of historical significance.
From an engineering standpoint I would suggest that it is not the strongest frame to put to motor use but if the tank were to become a structural member it would go a long way toward curing the inherent weakness issues of this frame design. The fork is probably best used by careful riders weighing in at less than 120 lbs or holding its own on a show bike.
I hope the above is helpful to you in determining what you would like to do with the bike.
Season’s best to you and have fun with the bike whichever route you choose to go.
Phil
I am almost afraid to start this post here due to the amount of quality builds. I want to build something that resembles a boardtracker total build cost $300.00 or less. I will start with a Zoombicycles 66cc 2 stroke kit 115.00 (shipped to my door) and this bike Girls, 5 bar Monarch serial # A50796 (freebie). I would like to know what year it is if anyone can help. I want to be able to ride it buy April. I have come up with this rough idea. I will try to build the tank myself so I am trying to minimize the fabrication and welding while keeping a decent look. I am sure the final product will change. I have decent mechanical skills but very limited welding skills. I am off and building 
