May one fix a baby stroller?

A detached wheel: Is forbidden to be reinserted even lightly.[1] Is it Muktzah? The weal is not Muktzah. If the carriage is not currently useable, it is Muktzah. If the carriage is still useable, it is disputed whether it is Muktzah.[2] According to all if it was used before Shabbos in its broken state it is not Muktzah.

Replacing a bassinet with a regular stroller seat or vice versa: Those strollers which can also be converted to a basinet through removing the seat and fastening a bassinet in the place of the seat, are forbidden to be converted on Shabbos through the use of screws. However, if they are attached through axes or hooks it is allowed.[3]  If the bassinet clicks in then seemingly it is forbidden to do so on Shabbos because such an attachment is considered semi-firm.

May one attach a hood to baby stroller?

This may involve a building prohibition in attaching it, as well as an Ohel prohibition in opening the attachment.

Regarding the Building prohibition in attaching it: If it is attached through screws then it is forbidden to be done. If it is attached through hinges, then it is allowed to be attached.

Regarding if one may open the hood once it is attached: It is forbidden to be opened[4]. However, there are Poskim[5] which are lenient[6]. See “The laws of making an Ohel” for further details.

May one insert or remove the seat of a baby walker?[7]

Yes, as it is meant to be constantly removed and inserted.

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[1] Due to a Rabbinical decree that one may come to insert it firmly and transgress a Biblical prohibition of Building:Tikkun Keli. [Based on ruling of  Admur:Rama in summary above, and so rules Ashel Avraham Mahdurah Tinyana; Betzeil Hachachmah 6:123, Az Nidbaru 8:33; Imreiy Yosher 1:202; SSH”K 15:77] However, according to the Chazon Ish [mentioned above by the summary] it is permitted to put it in loosely

[2] Az Nidbaru 8:33 rules it is not Muktzah. Kana Bosem 1:19; SSH”K 28 footnote 105 rule it is Muktzah.

[3] SSH”K 28:44

[4] SSH”K 24:13

[5] Az Nidbaru 3:24

[6] Seemingly they hold that once it is attached it is considered like a door and is thus allowed to be opened.

[7] SSH”K 28:46